Philosophy is, approximately, 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. Philosophy studies such concepts as existence, goodness, knowledge, and beauty. It asks questions such as "What is goodness, in general?" and "Is knowledge even possible?" Some famous philosophers have included Plato, Aristotle, Rene Descartes, John Locke, and Immanuel Kant. The following article explains some of the thinking behind this very imperfect, unrigorous, yet platitudinous account of philosophy.
Popularly, the word "philosophy" is often used to mean any form of wisdom, or any person's perspective on life (as in "philosophy of life") or basic principles behind or method of achieving something (as in "my philosophy about driving on highways"). That is different from the academic meaning, and it is the academic meaning which is used here.
Originally, "philosophy" meant simply "the love of wisdom." "Philo-" comes from the Greek word philein, meaning to love, and "-sophy" comes from the Greek sophia, or wisdom. Originally the scope of philosophy was all intellectual endeavor. It has long since come to mean the study of an especially abstract, nonexperimental intellectual endeavor. In fact, philosophy is itself a notoriously difficult word to define; the question "What is philosophy?" is itself, famously, a vexed philosophical question. It is often observed that philosophers are unique in the extent to which they disagree about what their field even is.
Philosophy vs. natural science
On the view of some in the (loosely described) Anglo-American philosophical tradition and the closely-related tradition of the Vienna Circle, philosophy ought to emulate the exact methods of science and mathematics. But strictly speaking, philosophy is to be distinguished from science. It is not, at least, part of philosophy to do experiments. Experiments play little, if any, role in the solution of philosophical problems. Someone might object to this, if he knows much about the intersection of philosophy and science. Philosophers are often referring to and interpreting the scientific work of physicists, who do experiments about space and time and quantum mechanics (see philosophy of physics). They are often referring to experimental work done in psychology when they discuss philosophy of psychology (see philosophy of psychology). In general, many philosophers who study philosophy of science are trained scientists.
There is no doubting that philosophers sometimes interpret and refer to experimental work of various kinds--especially in the philosophies of the so-called "special sciences," as in philosophy of physics and philosophy of psychology. But this is not surprising: the purpose of those branches of philosophy, branches like philosophy of physics, is to help interpret the philosophical aspects of experimental work. It is not the philosophers, in their capacities as philosophers, who do the experiments and who formulate explanatory theories of experimentally-tabulated facts.
There is a basic historical reason why philosophy is not experimental. Originally, the scope of philosophy was all abstract intellectual endeavor. Even up to early modern times, the people we now call "scientists" were referred to as "natural philosophers," i.e., philosophers who study nature. Over the years--it is very commonly observed--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, psychology has spun off.
One might wonder why scholars began to treat various special sciences as independent from philosophy. The answer for any given branched-off science is that it began to be prosecuted using rigorous, agreed-upon 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 one’s armchair, surrounded, at most, by some books and articles that scientists (but certainly other philosophers) write.
Of course, philosophy is far from being totally non-observational or non-empirical. Certainly philosophy makes essential use of 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 can require careful attention. But most (not all) philosophical topics require no more specialized knowledge than the average educated person has, except for specialized knowledge about philosophy itself (such as philosophical jargon).
Some beginners confuse philosophy and psychology, yet these are different fields. Philosophy does study the mind, just as psychology does, but it also studies other things besides the mind, too, about which psychology has nothing to say. The ways philosophers and psychologists study the mind differ, as well. The study of the mind involved in doing psychology involves careful, specific observation of particular mental phenomena, and experimentation; by contrast, philosophers think about more general aspects of the mind, questions like, "What is consciousness?" and "What is the relation between mind and body?"
Philosophy vs. religious studies and classics
What distinguishes philosophy from religious studies, most of which also is not experimental? Parts of theology, those which ask about what God is and how to prove that God exists, clearly overlap with what philosophers call "philosophy of religion." That is 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. That is not a problem either. Neither of these overlaps muddies the concepts of philosophy, of religious studies, or of classics.
But consider that other part of religious studies, the empirical part, which often focuses on comparative study of different world religions. 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.
Philosophy vs. mathematics
Mathematics differs from philosophy for other reasons. It uses some very specific, rigorous methods of proof that philosophers sometimes (only rarely) try to emulate, but rarely, if ever, duplicate with the same degree of rigor. As a result, mathematicians hardly ever disagree about results, while philosophers of course do disagree about theirs. Besides, most philosophers do not even try to make their work rigorous in a mathematical sense. Unlike mathematicians, philosophers disagree about their methods, and their methodological differences can often be used account for their different conclusions.
Another 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 usually does require hard labor and concentration, but at least a philosopher can often explain his question, with not too much difficulty, to an intelligent nonphilosopher in under ten minutes.
Some tentative generalizations about what philosophy is
So philosophy, it seems, is a discipline that draws on knowledge that the average educated person has, and it does not make use of experimentation and careful observation, though it may interpret philosophical aspects of experiment and observation.
More positively, one might say that philosophy is a discipline that examines the meaning and justification of certain of our most basic, fundamental beliefs, according to a loose set of general methods. But what we might mean by the words "basic, fundamental beliefs"?
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. It also studies value--the goodness of things--in general. Surely in human life we find the relevance of value or goodness everywhere, not just moral goodness, though that might be 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. (If indeed 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. 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.