I have noticed an apparent confusion when posing and answering questions as to what constitutes evidence in philosophy. Especially in scientifically-related areas, I often cite scientific evidence that I think is relevant. Sometimes, I get the impression that this is not considered necessary or sufficient. So what is philosophical evidence?
There is no canonical answer to this question, because the question of what constitutes evidence is an open question with multiple metaphysical positions. Evidentialism as an epistemological theory, like many positions in epistemology, depends on one's broader metaphysical commitments. From WP:
Evidentialism is a thesis in epistemology which states that one is justified to believe something if and only if that person has evidence which supports said belief.1 Evidentialism is, therefore, a thesis about which beliefs are justified and which are not.
Thus, there is no one "right" answer to this question, but rather a series of positions. The IEP treats the topic of evidentialism with more rigor, and rightfully makes manifest that there are important differences between scientific views on evidence and those that are salient in a philosophy of religion. From the IEP:
it is worth noting that evidentialism is also a prominent theory in the philosophy of religion. Evidentialism in the philosophy of religion has its own set of controversies, but this entry will not cover them. On evidentialism in the philosophy of religion, see Alvin Plantinga’s classic article, “Reason and Belief in God.” For a more extended discussion, see Plantinga’s Warranted Christian Belief.
So, while your question cannot be answered with a single position, it is possible to narrow the scope of your question by specifying which metaphysical presuppositions apply. Evidence in criminal justice? Evidence in Christian apologetics? Evidence in paranormal research? Chemistry research? And so and such like.
The short answer: It is the feeling to be familiar with the question at hand and to know the answer by intuition.
Evidence and intuition - as the corresponding capability to recognize matters of fact by a capability which remains unconscious - are related to each other. Evidence is a subjective certainty, and intution is the method to produce without much ado such certainty.
Intuition may be helpful as a heuristic. But it is not suitable to decide a philosophical discussion. A good example is the discussion we had some days ago at this forum concerning evidence-based Christianity. What seems evident to one group of persons may be even negated by another group.
The philosophical method relies on arguments and their assessment.
I was once a science student taking philosophy courses. I suspect your challenge isn't in understanding what "evidence" is, but in understanding what is going on in philosophy discussions.
In the (undergraduate) science classroom, students are principally occupied with learning some set of facts and relationships. These facts are expected to be supported by evidence, which comes in the form of empirical observation and analysis
Philosophy discussions do not work this way. It might be easier to think of philosophy as an exploration of ideas. Instead, you are focusing on the logic, rigor, and clarity of ideas. Participants expected to understand and dissect arguments. There is no "evidence", only logical argumentation.
As a fellow wayfarer, journeying through the philosopher's world, some parting strategies to enhance your participation:
- Arguments should be logical. That is, the conclusions should follow from the argument provided. Look for cases counter-examples where the reasoning doesn't hold.
- Arguments should be clearly articulated. Many problems arise when language is unclear, terms are used inconsistently, etc. Question what key terms mean.
- Arguments should be explicit. Look for hidden assumptions. Exposing when the author's (or speaker's) argument requires an unstated assumption can advance a conversation by pointing out previously hidden nuances - or weaknesses.
- Arguments should be consistent. When a person contradicts themselves, it should raise a red flag. Does their argument clearly support treating those cases as different things, or does it indicate a weakness in their reasoning?
I think of philosophy as the primordial soup in which other fields of human endeavour are formed. The end goal of a philosophical discipline is to "graduate" to be its own field.
Science, itself, used to be thought of (and called!) "natural philosophy". Logic was once a wholly philosophical discipline, and now it's gone through mathematics and computer science and is now (in part) engineering. We actually build things entirely out of logic!
The answer to your question, therefore, is that it depends. Moreover, a large part of philosophy is trying to answer the question of "what constitutes evidence" or "what constitutes sound methodology" in a specific situation.
Having said that, there are experimental parts to philosophy, and the normal rules of scientific evidence applies. The ur-example is in moral philosophy, where you can run experiments to try to work out if there is a consistent sense of "morality" in some hypothetical model scenario (e.g. trolley problem) across cultures.
When it comes to philosophical conceptual analysis, a common practice is to advance a general/universal definition and then "test" the definition by trying to come up with particular counterexamples. How is an example identified as a counter? If it matches the proposed definition but "intuitively" doesn't match the concept being defined.
But then you will find a debate between those who weigh the intuitions of so-called experts over the intuitions of the philosophical "laity." The latter weighing is the sine qua non of so-called experimental philosophy.
Another strategy is transcendental argument. Roughly (very roughly), this works from the possibility of an evidence type to necessary principles of the type (or that is a way to interpret the practice, anyway).
Quine's naturalized epistemology partly gave birth to another kind of philosophical "evidence," i.e. indispensability arguments. Those are akin to transcendental arguments but in a sense draw more on a posteriori considerations. The classical example concerns a surprising realism about mathematical objects. As the IEP article puts it:
The indispensability argument in the philosophy of mathematics, in its most general form, consists of two premises. The major premise states that we should believe that mathematical objects exist if we need them in our best scientific theory. The minor premise claims that we do in fact require mathematical objects in our scientific theory. The argument concludes that we should believe in the abstract objects of mathematics.
One might view an indispensability argument as based on the theory of ontological commitment:
On its face, the notion of ontological commitment for theories is a simple matter. Theories have truth conditions. These truth conditions tell us how the world must be in order for the theory to be true; they make demands on the world. Sometimes, perhaps always, they demand of the world that certain entities or kinds of entity exist. The ontological commitments of a theory, then, are just the entities or kinds of entity that must exist in order for the theory to be true.
Yet another sphere of philosophically informed theories of evidence is the dialectic of higher-order evidence. To try to bring things full circle: questions of meta-evidence might be taken as questions of epistemology, of the analysis of knowledge, and we can imagine that first-order evidence for theories of second-order evidence would be conceptual analyses of the latter.