It has been said that biological organisms are one kind of machine, albeit highly complex ones. But is this really true? To answer this question, one needs a precise definition of "machine". So, is there a definition of machines in some paper or book, that can tell whether humans and other animals are machines?
I like this question. It's thorny.
Merriam-Webster defines machine so: a mechanically, electrically, or electronically operated device for performing a task.
That is, there is an operator (the entity that will perform the task) and there is a purpose (the task). And the machine is a means by which to perform the task. Various other dictionaries all have variants on this. A machine is some kind of mechanism whereby some task is performed or some physical purpose is achieved.
Can a biological organism be reasonably said to satisfy such a definition?
The usual definition of a biological living thing is (tersely) irritability and reproduction.
Here, irritability is a technical term meaning that the organism can respond in some way to changes in its environment. Some way other than simple physical response. If you put water on clay, it gets wet. If you put water on a seed (a seed in the right conditions etc.) the seed will start to grow. Thus, living things respond to changes in their environment by proceeding with their living processes.
Reproduction means, of course, making some kind of not-necessarily-exact copy of themselves. Possibly by splitting in two, or possibly through sexual reproduction, or some other such means.
The organism attempts to achieve the result of making copies of their DNA. (For DNA based living things. Let us set aside the additional thorny question of whether RNA based viruses are living things.)
In the book The Selfish Gene, Dawkins uses the metaphor of genes having the desire to reproduce. He goes to pains to point out that he is not literally saying they have desires, but that genes that produce results that tend to make copies of themselves will tend to become more common.
So, by this metaphor, an organism is a machine by which a collection of genes attempts to achieve the result of making copies of themselves. They don't literally have desires, but treating the situation as though they do is a useful method of analyzing the situation. It allows predictions/explanations of things that would otherwise be difficult to predict or explain.
And this metaphor also exposes several other thorny questions. Another term for the the metaphor is "intentionality." That is, if we imagine a thing has intentionality, can we predict its behavior in useful ways? Often yes. In some cases (a tiger wanting to eat us) it seems quite reaonable to suppose the thing really has intentionality. And it guides you in avoiding getting eaten. In others (a storm wants to blow away your roof) it is possibly a bigger stretch to suppose the storm really has intentionality. But it still lets you make your roof more secure.
So, at least as a metaphor, organisms can be said to be machines used by genes (the operators) for the purpose of making copies of themselves (the task).
One of the (many and varied) definitions of "Machine" in the Oxford English Dictionary is:
A structure regarded as functioning as an independent body, without mechanical involvement.
Humans and animals are self-evidently machines by that definition, unless you believe that we are controlled by some external entity pulling our strings.
- A living body, esp. the human body considered in general or individually. Now chiefly figurative from sense 6b (cf. sense 8b).
6 b. A complex device, consisting of a number of interrelated parts, each having a definite function, together applying, using, or generating mechanical or (later) electrical power to perform a certain kind of work (often specified by a preceding verbal noun).
which seems to answer the question clearly in the affirmative.
8b is the sense of a person acting without volition (c.f. automaton)
A lot of problems in philosophy seem to boil down to differences in the meanings of words - perhaps we need a less ambiguous language?
This question is fundamentally about the branch of philosophy known as Ontology, which is broadly concerned with questions of categorization (among other related things such as being, becoming, and reality). Definitions and classifications are tricky things that are often too restrictive or too loose to be useful. I'm reminded of the classic story of Diogenes countering Plato's definition of a man as "a featherless biped" by plucking a chicken and exclaiming, "Behold! A man!"
In any case, definitions are where most people think to start, so there is value in starting our discussion there, but then I think we need to move into the more philosophical questions of "what does it mean to be human?" (Ontology) and **"why should/shouldn't we consider living things to be machines?" (Ethics).
Is there a definition in some paper or book...?
The other answers here start with dictionary definitions. Dictionaries are not the best starting point for developing an ontological argument. An important thing to remember about dictionary definitions is that they are largely descriptive, rather than prescriptive: new definitions are constantly being added and older definitions marked as obsolete or rare as they fall out of usage with the people who speak the language. The online version of the Oxford English Dictionary maintains many of these older definitions for words in the English language. From there you can see that the word "machine" has had, and continues to have, a variety of definitions dating back to the 16th century. Some of these definitions, such as
I. A structure regarded as functioning as an independent body, without mechanical involvement.
I.2. A living body, esp. the human body considered in general or individually.
obviously would classify human bodies as machines because it is right there in the definition. Other more modern definitions, such as
IV. An apparatus constructed to perform a task or for some other purpose; also in derived senses.
IV.b. A complex device, consisting of a number of interrelated parts, each having a definite function, together applying, using, or generating mechanical or (later) electrical power to perform a certain kind of work (often specified by a preceding verbal noun).
Are less clearly applicable to humans. Other answers have used these definitions in their arguments, but these definitions only describe how the word "machine" has been used. They are not intended to form the basis of an ontological system that classifies some things as machines and other things as not-machines because the usage of the word machine is very broad in the English language.
Famous Philosophical Definitions
"Fine," you might say, "so then what about a book by some famous philosopher that defines machine?" Sure, you could do some reading and see if any famous philosophers have discussed machines in their work.
The first discussion of machines that comes to mind are the simple machines of Archimedes, which were later expanded by other ancient Greek thinkers to include levers, screws, pulleys, inclined planes, wedges, and the wheel and axle. Then complex machines were simply assemblages of the simple machines (like a block and tackle being made of many pulleys). Modern machine analysis in engineering usually considers kinematic pairs instead. A biomechanical analysis of the human body shows how kinematic pairs such as revolute and spherical joints are present in the human body. Does this mean that the human body is a machine? We've found tiny insects with gears in their joints as well! Does that mean these insects are complex machines?
The second and far more recent definition of machine that comes to mind for me is a Turing Machine, which has been a foundational concept in mathematics and modern computer science. However, the purpose of the Turing Machine concept as it was defined by Turing was to answer theoretical mathematical questions regarding the solvability of certain classes of mathematical problems given a very specific process for executing the computations. Someone more familiar with the mathematics than me might be able to build an argument for why human thought could be modeled by a Turing Machine. However, I'm not sure this is the definition you are looking for either.
There are a wide variety of famous philosophers who could each have their own definition of a machine. Perhaps other users on this site can find more relevant philosophical definitions than the examples I give here. However, much like dictionary definitions, they are dependent on context. A lever is a machine according to Archimedes, but is it a Turing Machine? Which definition is the "right" or "true" one? If starting from the definition, whether dictionary or otherwise, of machines is not a good way forward, what is?
Ontology and Human Being
One question that has interested philosophers for a very long time, if not all of human history, is the nature of what it means to be human. What are the distinctive features of a human? What separates humans from other living things? What separates living things from non-living things? I think this is the true crux of your question: what really separates a non-living thing from a living thing (biological organism in your question).
This question is so broad that it is practically impossible to cover all the possible viewpoints of philosophers over human history in a single answer on this site. Some assert that humans have souls and bodies. Others would say instead that humans have minds and bodies. Some would say that there is nothing to existence except for material things, so there is no immaterial soul or mind outside of or separate from the body.
Are human beings just complex machines (bodies) being operated by a soul or mind behind the figurative steering wheel? If the human body is a machine, is that sufficient evidence to say that a human is a machine? If a human mind can be simulated by a Turing Machine does that make a human a machine? The answers really depend on the school of thought that you subscribe to. You can feel free to go down the rabbit hole with some the links I've shared to learn about the many philosophical schools that tackle questions about mind, body, soul, and what it means to be human.
When Ontology Becomes Ethics
It would be remiss of me to finish a philosophical discussion without asking "WHY?" To what end do we classify humans and other living things as machines? Why is important that we do not classify humans and other living things as machines?
I will say that one crucial aspect about my intuitive understanding of the concept "machine" is that machines are objects, not subjects. Objects are usually not considered as having moral agency or being worthy of moral consideration. They can be owned, bought, sold, divided, destroyed, modified, and otherwise acted upon without requiring consent or consultation.
Immanuel Kant was one philosopher who was pretty clear about the necessity of treating humans as ends in themselves, and not a means to an end. Human beings have been, and continue to be, objectified through institutions including slavery, colonialism, patriarchy, and capitalism. (Side note: this is not to say that Kant was against all of these things.) Humans have always treated other living things as a means to an end: treating plants and animals as food, fuel, clothing, building materials, and/or obstacles to economic development, resulting in massive ecological devastation and species extinction around the globe.
So I will leave you with that thought: what does it mean for our worldview when we start to classify living things as machines?
I feel like in order to be useful, we can't just define a "machine", we have to also define what is "not a machine". By some definitions it seems everything could be called a machine. The entire universe may be a machine.
But I think the crux of the question boils down to: machines are things that can be mapped out and understood with present science and technology.
Coffee maker? Machine. We fully understand it. We can mass produce them. Jet airplane? A lot more complex but definitely still a machine. Trees? Now we're getting into the interesting parts of the argument because it could be argued that trees are composed of parts, each serving specific functions, and therefore a tree is a machine, following the laws of nature/physics (and thus towards the path of "everything is a machine if you get right down to it").
So I propose two counter-arguments:
We can't call it a machine if we, as a society, are incapable of understanding how it works. Maybe humans are machines but we can't declare that when we don't fully understand how they work. Reference modern efforts at artificial intelligence. It's still a long way from producing anything that actually can act and operate as a human. We don't understand the process well enough. So maybe humans actually are machines but we can't really say that because the truth is we don't know all the details.
Machines must be deterministic. When you're asking if humans are machines or not, I think the implied question is that machines are deterministic: the axles, cogs and parts determine how the machine will run. If humans are machines, then we must be deterministic: bound by the laws of nature and physics; free will does not exist.
But I think there is actual evidence contrary to this, namely that quantum mechanics is non-deterministic. It may be impossible to fully understand or simulate a human brain due to this non-deterministic nature at the most fundamental level. In that event, we cannot be considered to be machines. (You could take this argument to extremes too, and say that since the universe is built on non-deterministic quantum mechanics, even machines aren't truly machines -- on a macro level they can appear to be but if you fine-tooth the detail far enough, all the way down to the quantum level, then they are no longer entirely deterministic.)
I think here suits terminology from a computer science. State machine is a device which switches between internal states in repetitive ways (cycles).
In this perspective, humans are machines. One of many our cycles : Sleep - Eat breakfast - Go to Work - Work - Go Home - Eat - Watch Tv - Go to sleep. Next day - all over again, maybe with slight modifications.
Or machinery at the microscopic level in our cells : Burn calories - Divide - Emit toxins into the fluids. And same again, until cell death. So bigger machinery can be constructed from a smaller machinery sets.
I don't think that there can exist such a unique creature which could switch internal states only in a unique and non-repeatable pattern. Even "broken" machines (mentally ill people for example) has inner triggers of acting one way or another.
For acting in a complete unique way, you must have some "device" in your brains which evaluates uniqueness of each and every action, which, alas, by definition is yet another machinery. Because such introspection would have clear rules set of how to filter out this or that non-unique action.
So, no way out. Once machine has been created, - it will stay so until the end. Like it or not.
My gut feeling is that the other answers are carefully avoiding the elephant in the room: The profound feeling that there is more to us (and to a lesser degree, perhaps, to other organisms as well) than our material appearance.
Religions are the most obvious expressions of this: They claim that humans are linked to a transcendental divine being that imbues them with something special that eludes scientific approaches, a soul, if you want.
Even if one doesn't buy into spirituality in one of its many flavors, humans undeniably are quite special among the machines. The following list of distinctions applies, to a lesser degree, to (some) animals as well.
- Humans are self-conscious and self-modifying.
- Humans are not operated, but instead they are operators.
- We don't serve a purpose (which, as an aside, is opposing the typical religious tenets) but we use other things for purposes of our own.
- In a way, we are meta-machines: We design and build other machines.