This is subject to debate and there is no definite answer. The general consensus is that no definite set of properties can possibly be given and if it is done, these sets are relative to the end they serve and the historical as well as the cultural context, i.e. we live in a "post-essentialist world" (Ramsey 2013) when it comes to the definition of species. Another aspect that should be held in mind is that defining human nature by essential properties means that any individual that misses any of these properties seizes to be "human", i.e. there are severe ethical consequences of how we conceive "being human". I can and will give you an oversight of positions regarding the subject matter and point you towards relevant sources found at the end of the answer.
(These textbits have been part of my MA dissertation and are mostly a direct copy of my own intellectual property)
What does it mean to be an essential property of humans?
David L. Hull gives the following definition for essentialism regarding a biological species:
[A] biological species might be characterised by one or more characters [or properties/traits] which are both universally distributed among and limited to the organisms belonging to that species (Hull 1986, 3)
Thus, essentialism in the context of "humans" means that there is a set of properties that are both universally exhibited by humans, i.e. true for all humans, and distinct for humans, i.e. not true for any non-human individual.
Additionally, essential properties have historically been thought to be intrinsic as well. As the linked SEP article points out, this is not necessarily so, but especially in the context of the "essentially human", most approaches assume that the properties have to be intrinsic in order to be essential. As the discussion of Taxonomical Relationalism below shows, there are critical voices against this understanding.
Empirical arguments against an essential human nature: The Darwinian Challenge
The often discussed locus classicus of the Darwinian Challenge is the paper On Human Nature (Hull 1986) by David L. Hull. He argues for a complete omission of any scientific account of human nature defined by traits or properties of a species. Instead, one should simply use a genealogical understanding of species, i.e. define them by family relations (Ramsey 2013; Kronfeldner, Roughley & Toepfer 2014). The summary of Hull’s argument given by Ramsey (Ramsey 2013) suggests a twofold argument: On one hand, biological species are as a matter of fact defined by “evolving lineages” (Hull 1986:10) in the science of biology, i.e. by family relations, and not by traits or properties exhibited. On the other hand, there is no set of traits exhibited by every member of a species at a given time or throughout history that can be considered as unique to that species, i.e. a set of properties both universal and distinct for the species. That both sides of the argument reinforce each other cannot be understood from Ramsey’s summary alone, but only by adding a crucial third argument of Hull’s: both genetical and phenotypical variability are indeed essential (sic!) and necessary features of complex biological species since they are necessary for their evolutionary success. This, Hull argues, is particularly true for the species homo sapiens (ibid:10–11). Thus, biological kinds are fundamentally different from natural kinds as they have to show a great variety in their instantiations.
So regarding an essential human nature, the addition of Hull’s third argument suggests that it is in the very nature of our species to show a great variety of traits. Consequently, there is no single set of traits both universal and distinct for humans, in turn resulting in family relations being the only viable option for defining this species.
In other words, every possible definite set of traits used to define the essential nature of humans leaves us with a dilemma:
One possibility to keep it universal and distinct is to make it so restrictive that we exclude individuals that are in fact part of the biological species (i.e. direct relatives of humans) simply because they lack at least one trait used to define "essential" human nature. For example, if we consider meaningful expression as essential to being human, a person permanently falling into a coma due to brain damage may simply seize to be human because she cannot express her will and answer rationally to demands anymore. This also implies that this "former" person seizes to have the ethical and legal status of a human, i.e. becomes a mere "thing".
The other possibility is to make it so lenient that individuals of other biological species would have to be considered "human", i.e. we can cover all individuals that we commonly consider human, but as a side-effect the set of properties is only insofar "distinct" as we simply include animals of other biological species and label them "human". This would also mean that "human rights" apply to them.
In other words: A set both universal and distinct for the biological species homo sapiens is nigh impossible. As if this would not be bad enough, one has to conclude that even if, against all odds, a set of traits that is really universal and distinct for the biological species homo sapiens would be found, it would be a mere historical contingency. Or as Edouard Machery puts it:
To focus on humans, it is hard to find any property that is both distinctive of humans and common to all humans. Furthermore, even if a property were both distinctive and universal, this state of affairs would be contingent. It would not be a necessary property for being a human. (Machery 2008:325)
As we will see in the upcoming discussion of the Darwinian Challenge, those criticisms as well as most answers have two things in common:
Firstly, the argument is broadly successful as they tend to acknowledge that the implications of a scientific understanding of evolution and essentialist notions so understood are antithetic to one another.
Secondly, the answers to this “Darwinian challenge” to human nature revolve around traits or characteristics that can be grasped through the methods of the empirical or natural sciences as well, i.e. do not leave the boundaries of the discourse as a scientific one. In other words, they try to dispel a biological argument while remaining in a biological framework.
Possible answers to the Darwinian Challenge
An extensive overview of the possible reactions to the Darwinian Challenge is given by Kronfeldner et. al. (Kronfeldner, Roughley & Toepfer 2014). If the Darwinian Challenge is to be dealt with constructively, neither rejecting the scientific insights and evidence put forward and returning to classic essentialism nor simply accepting pluralism are viable options. This leaves us with four different approaches as per Kronfeldner et. al.:
The constructive positions held in the recent literature attribute different epistemic roles to the concept of human nature: descriptive (descriptivism), explanatory (explanativism), taxonomic (taxonomic relationalism), or a new combination of explanatory and deﬁnitional roles (property cluster essentialism). (ibid:645)
Descriptivism acknowledges the argument insofar as it does not argue that any given set of traits serves both aspects of the definition of natural kinds, i.e. can play both a definitional and explanatory role. This approach omits any explanatory ambitions and still serves the scientific need for an accurate description of how the world is and a definition of the explanandum. For purely practical considerations, it delivers sufficiently accurate means to determine the object of scientific inquiry into the human. There are two general ways to do so: Firstly, one may try to find traits that are generally exhibited by members of the class described. This constitutes a conceptual grasp that does not even attempt to be a description of a unique "nature" that also allows for taxonomical use since many of the properties used are not unique to the species (e.g. Machery’s nomological approach). The second way specifically uses trait clusters instead of one single set of traits. This allows for more accuracy regarding the description of the uniqueness of the human at a given point in time (e.g. Ramsey’s approach of using historical trait clusters). Here, the historicity still forbids taxonomical use in the sense of biological species but overcomes the lack of specificity.
Explanativism takes a similar route as descriptivism insofar as it also omits one part of what natural kinds are supposed to allow for but instead concentrates on the other aspect of the definition of natural kinds, the explanans. While it also refuses to call any descriptive cluster "human nature", explanativism starts considerations with descriptive determinations as that which is to be explained by human nature. The outcomes are patterns or explanative categories that are not themselves directly observable features of humans. Thus, they cannot be used in a descriptive sense but are supposedly necessary conditions for or explain the specific similarities and differences within the human species and between species as caught by descriptive approaches, i.e. they “provide the deepest explanation of why humans are the way they are” (ibid:647). These approaches can take a variety of guises, but I will focus on those mentioned by Kronfeldner et. al.:
The comparative studies of apes against human children by the research group around Michael Tomasello resulted in his hypothesis of a shared intentionality. This kind of behavioural relation to the world and other members of the species is shown to be unique to and typical for humans and used to explain many behavioural patterns and capacities specifically human (Tomasello 2008). This approach explicitly refers to the structure of human communication but treats it as something relying on and built upon capacities and structures that developed through evolutionary processes that further explain cultural and linguistical diversity (ibid:7–12).
Richard Samuels analyses cognitive and behavioural sciences as relying upon and trying to model mechanisms that allow for descriptions and especially predictions of human behaviour. Thereby, he identifies mechanisms that account for both similarities and differences exhibited by causing these similarities and differences. A specific element of his theory missing in the elaboration by Kronfeldner et.al. is his adoption – and defence of the use – of “HPC kinds” (HPC = homeostatic property cluster) developed by Richard Boyd. HPC kinds modify the definition of natural kinds so that they do not require intrinsic properties anymore, among other things, but still can serve both definitional and explanatory deeds (Samuels 2012:21–24).
Denis Walsh proposes that those capacities and mechanisms that allow for evolutionary development of a species constitute explanatory essences. As Samuels points out, this is an interpretation of Aristotle’s writings on causation that adopts some views on how the coming into being of a thing is that which makes it the way it is (ibid:20). Walsh indeed applies his argument not only on human nature, but is rather trying to argue that a certain Aristotelian understanding of essence is “indispensable to evolutionary biology” in general (Walsh 2006:425): those explanatory essences are necessary capacities and mechanisms that explain how evolution is possible.
The theory of Kim Sterelny identifies two different feedback loops that are both unique and typical to humans and explain their behavioural variety and evolutionary potential. Therefore, it can be seen as an approach similar to Walsh’s in describing explanatory essences that focus on explaining the particular evolutionary becoming. On the other hand, Sterelny's focus is learning and social behaviour and he tries to explain the unique human abilities in this field as having developed through evolution, hence he is probably closer to Tomasello in both function and focus of his concept of positive feedback loops.
Taxonomic Relationalism focuses on how the boundaries between species are drawn within science: on one hand, the method of using genealogical relations is declared as the primary means to distinguish biological species and families, on the other hand, this primary method is complemented by descriptive means for determining a particular species as an object of inquiry where the methods are not in conflict. In that view, intrinsic properties can never define individuals as members of a biological species as they result from relational properties as the only possible basis of any scientific taxonomy. In this sense, relational properties are explanatory for intrinsic properties. More specifically, Samir Okasha describes essential properties as being relational rather than intrinsic ones and thereby dodges the anti-essentialist criticism of Mayr, but also Kripke, Putnam, and Wiggins (Okasha 2002). By that, he goes beyond and in some ways against Hull, who suggests a similar approach.
Property Cluster Essentialism
Property Cluster Essentialism is mainly built on the HPCs developed by Richard Boyd. While Kronfeldner et. al. claim that Boyd mainly modifies the notion of essences so that they do not have to be necessary, sufficient, and intrinsic (Kronfeldner, Roughley & Toepfer 2014:648), Boyd himself argues for the necessity of modifying the understanding of natural kinds to accommodate biological species, namely by dropping lawfulness, exactness, necessity and sufficiency, and ahistoricity/eternality (Boyd 1999:151–57). It is also interesting that Kronfeldner et. al. categorise Samuels under explanatory approaches while he himself argues for using HPCs because they can serve both explanatory and definitional needs. The reason probably lies in Samuels’ labelling of HPCs as causal essences, i.e. because of the assigned primary role they are meant to play in his argument. As Samuels points out, there is no way HPCs can play the taxonomic role of classical essences, while Boyd argues for their principle viability for that end.
The discussion ends with the same conclusion as the one drawn by Kronfeldner et. al.: there obviously are both possibility and need for concepts of human nature that can dodge or adapt to the Darwinian Challenge. The three main concerns are:
That despite the great number of different theoretical considerations and arguments for such a possibility, there are few – if any – takes that are worked out to a level that makes them directly applicable, especially regarding a direct link to empirical findings.
The approaches to render human nature will have to be accommodated to the particular needs of the respective application – which will differ between sciences and sometimes even within a single science, thus constituting a plurality of “human natures” without any means to rank them as more or less appropriate.
When considering human nature, the normative implications are seldom of interest for scientific authors, although they are of obvious practical importance when applying the concept.
Ethical arguments against essentialism
As already mentioned, there are also normative implications of assuming a property as being essential for being human: In common language talk, there is no problem with calling it essential. The colloquial meaning just implies that what "really" makes us human is, for example, searching for meaning, right? But in the technical sense, an implication of an essential property is that any individual that does not have this property simply does not belong to the class described by this property as an essential one. So humans not able to search for meaning would have to be considered not human.
So if pre-linguistic children, mentally impaired, elderly people with severe dementia or comatose people do not, not yet, or not anymore have the cognitive capacity or means to strive for meaning and this is considered essential for being human, the only possible conclusion is that they are not humans. And if they are not humans, there is no obvious reason to treat them as humans - which runs against our moral intuitions.
In other words: While the search for meaning is probably a property distinctly human and one that is generally exhibited by humans, there are good ethical reasons to reject it being essential since the few exceptions should still be considered human without having this property, after all.
Albers, Marion, Hoffmann, Thomas & Reinhardt, Jörn (eds.) (2014): Human rights and human nature. Dordrecht: Springer.
Boyd, Richard (1989): 'What realism implies and what it does not', Dialectica 43:1‐2, pp. 5–29.
Boyd, Richard (1991): 'Realism, anti-foundationalism and the enthusiasm for natural kinds', Philosophical studies 61, pp. 127–148.
Boyd, Richard (1999): 'Homeostasis, Species, and Higher Taxa', in: R. A. Wilson (ed.), Species: New Interdisciplinary Essays. Cambridge, Massachusetts: MIT Press, pp. 141–186.
Ereshefsky, Mark (2009): 'Natural kinds in biology', in: E. Craig (ed.), Routledge Encyclopedia of Philosophy. New York: Routledge.
Ghiselin, Michael T. (1997): Metaphysics and the origin of species. Albany: State Univ. of New York Press.
Hull, David L. (1986): 'On Human Nature', PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association 1986:2, pp. 3–13.
Kronfeldner, Maria, Roughley, Neil & Toepfer, Georg (2014): 'Recent Work on Human Nature: Beyond Traditional Essences', Philosophy Compass 9:9, pp. 642–652.
Machery, Edouard (2008): 'A Plea for Human Nature', Philosophical Psychology 21:3, pp. 321–329.
Okasha, Samir (2002): 'Darwinian Metaphysics: Species and the Question of Essentialism', Synthese 131:2, pp. 191–213.
Ramsey, Grant (2013): 'Human Nature in a Post-essentialist World', Philosophy of Science 80:5, pp. 983–993.
Roughley, Neil (ed.) (2000): Being Humans: Anthropological Universality and Particularity in Transdisciplinary Perspectives. Berlin: De Gruyter.
Samuels, Richard (2012): 'Science and Human Nature', Royal Institute of Philosophy Supplement 70, pp. 1–28.
Sterelny, Kim (2012): The evolved apprentice: How evolution made humans unique. Cambridge, Massachusetts: MIT Press.
Tomasello, Michael, et al. (2005): 'In Search of the Uniquely Human', Behavioral and Brain Sciences 28:05.
Tomasello, Michael (2008): Origins of human communication. Cambridge, Mass.: MIT Press.
Walsh, Denis (2006): 'Evolutionary Essentialism', The British Journal for the Philosophy of Science 57:2, pp. 425–448.