Consider first how to distinguish philosophy from science -- from disciplines like physics and chemistry. Well, itís not part of philosophy to do experiments. Experiments play little, if any, role in the solution of philosophical problems. Now someone might object to this, if he knows much about the intersection of philosophy and science. He might say, "But philosophers are often referring to and interpreting the scientific work of physicists, who do experiments about space and time and quantum mechanics. And they are often referring to experimental work done in psychology when they discuss philosophy of psychology."
Thereís no doubting that philosophers sometimes interpret and refer to experimental work of various kinds -- especially in the philosophies of the different sciences. For example, in philosophy of physics, or philosophy of psychology. But thatís not surprising of course: the purpose of those branches of philosophy, branches like philosophy of physics, is to help interpret the philosophical aspects of experimental work. But at any rate itís not the philosophers, in their capacities as philosophers, who do the experiments.
There is a basic historical reason why philosophy is not experimental. Originally, "philosophy" meant simply "the love of wisdom." The "philo-" part comes from the Greek word philein, meaning to love, and the "-sophy" part comes from sophia, or wisdom. Originally the scope of philosophy was all abstract intellectual endeavor. Even up until early modern times, the people we now call "scientists" were referred to as "natural philosophers," i.e., philosophers who study nature. Over the years, the scope of philosophy has gotten smaller and smaller, as different sciences have spun off and become independent disciplines in their own right. Some relatively early "spin-offs" were physics and chemistry; more recently, just within the past 100 years, psychology has spun off.
So of course one might wonder how thinkers knew or sensed that a new discipline was to be treated as independent from philosophy. The answer is that the discipline began to be prosecuted using rigorous methods of observation and experimentation. Philosophy in its core sense, the sense that remains today, is essentially something that one should be able to do from oneís armchair, surrounded, at most, by some books that scientists write. But be careful thinking about this. I emphatically do not mean that philosophy is totally non-observational, or non-empirical. Certainly philosophy makes use of, in a really essential way, observations about the world. But they are, we might say, very general observations -- observations like "It seems to me I make free choices" and "It seems to me that killing another person, if ever necessary, requires a really good excuse." Observations like this take a great deal of investigation to make; they require careful attention. But most (not all) philosophical topics require no more specialized knowledge than the average educated person has; except perhaps specialized knowledge about philosophy itself.
So philosophy is not experimental and its observations are only very general, broad observations. And that is what makes it different from natural sciences like physics, and social sciences like psychology. So mind you, some people confuse philosophy and psychology, but they are different. Philosophy does study the mind (and it also studies other things besides the mind, too), just as psychology does. But the study of the mind involved in doing psychology involves careful, specific observation of particular mental phenomena, and experimentation; philosophers think about more general aspects of the mind, questions like, "What is consciousness? What is the mind itself?"
All right, then what distinguishes philosophy from religious studies, which also is not experimental? Some parts of religious studies -- parts of theology, anyway -- clearly are philosophical. Parts of theology, which ask about what God is and how to prove that God exists, clearly overlap with what philosophers call "philosophy of religion." And thatís not a problem. Similarly, classics, or study of ancient Greece and Rome, studies the Greek philosophers Socrates and Plato, and so classics overlaps with an area of philosophy, namely history of Greek philosophy, to that extent. And thatís not a problem either. It doesnít muddy the concepts either of philosophy or of classics. But what about that other part of religious studies, the empirical part, which often focuses on comparative study of different world religions? Well, that part of religious studies can be distinguished from philosophy in just the way that any other social science can be distinguished from philosophy. Namely, it involves specific observations of particular phenomena, here particular religious practices, and philosophy does not.
So perhaps now you can see how philosophy differs from various of the sciences, psychology, and religious studies. But how does philosophy differ from mathematics? It certainly is not experimental.
Mathematics differs from philosophy for other reasons. For one thing, it uses some very specific, rigorous methods of proof that philosophy may try to emulate sometimes, but rarely, if ever, succeeds in duplicating. As a result, mathematicians hardly ever disagree about results. But philosophers disagree about their methods and their methods are not always nearly as rigorous as the methods mathematicians use. And dialectic, while surely a part of progress in mathematics, does not play the same sort of prominent role that it plays in philosophy.
A more straightforward way to distinguish philosophy from mathematics is this. Math, beyond a certain basic level, requires some extremely specialized knowledge, which can be obtained only by dint of extremely hard labor and concentration. It is not the sort of discipline that can be pursued with the knowledge that the average educated person has. Philosophy, to be pursued right, sometimes does require hard labor and concentration, but at least a philosopher can explain his question, with not too much difficulty, to an intelligent nonphilosopher in under ten minutes.
So Iím suggesting that philosophy is a discipline that draws on knowledge that the average educated person has, and does not make use of experimentation and careful observation; though it may interpret philosophical aspects of experiment and observation. OK, so this tells you what philosophy is not. The question now is what philosophy is.
But I have already suggested an answer to that question. Namely, it is a discipline that examines the meaning and justification of certain of our most basic, fundamental beliefs, according to a certain general method which I explained at length. If thereís anything that is unclear about this, it is, I think, what we might mean by these words: "basic, fundamental beliefs." Is there any way I might make that clearer?
Well, try this out for size. A belief is fundamental if it concerns those aspects of the universe which are most commonly found, which are found everywhere: the universal aspects of things. Philosophy studies, for example, what existence itself is. What could be more universal than that -- than existence itself? It also studies value -- the goodness of things -- in general. And surely in human life we find the relevance of value or goodness everywhere. I donít mean just moral goodness, though thatís very important, but even more generally, goodness in the sense of anything that is actually desirable; the sense, for example, in which an apple, a painting, and a person can all be good. (Well, if there is a single sense in which they are all called "good.")
Of course, physics and the other sciences study some very universal aspects of things; but it does so experimentally. Philosophy studies those aspects that can be studied without experimentation. And those are aspects of things that are very general indeed; to take yet another example, philosophers ask what physical objects as such are, as distinguished from properties of objects and relations between objects, and perhaps also as distinguished from minds or souls. Physicists proceed as though the notion of a physical body is quite clear and straightforward -- which perhaps in the end it will found to be -- but at any rate, physics assumes that, and then asks questions about how all physical bodies behave, and then does experiments to find out the answers. Philosophy just doesnít work that way.
With all of what Iíve said so far as background, Iíll offer a definition. I should make one more remark first, though: as philosophers disagree about everything, one of their points of disagreement is about what philosophy is. So donít go around thinking that the following is the generally agreed-upon definition of philosophy, because it isnít. What it is, is a well thought out definition which, I am confident, most philosophers would acknowledge as at least a good and plausible attempt. Anyway, here goes.
Philosophy is the study of the meaning and justification of beliefs about the most general, or universal, aspects of things; a study which is carried out not by experimentation or careful observation, but instead typically by formulating problems carefully, offering solutions to them, giving arguments for the solutions, and engaging in dialectic about all of the above.
Iím not going to tell you that this is a perfect definition, because Iím sure it isnít. But I think it should give you a pretty good idea of what "philosophy" means.