Philosophy is a disorder that causes people to ponder pondering, rather than go out and get a job. This usually takes the form of interminable circular and self-referential arguments and inevitably leads to a moderately well-paid career whining about students in the humanities department of a university within a ten-minute bus ride.
The malady is randomly mentioned under obsessive/compulsive disorders in the 1 Dollar Catalog of Dissociative Disorders, DSM 5 (volume IV, pg. 1546, section 172, lines 45-21). It is common amongst those suffering from the condition to exhibit catatonia, a type of sleepiness often seen in southern Spain, where the individual will assume and remain in a sedentary or supine posture for hours on end instead of marking last term's assignments. A classic and rather sad example is depicted in Rodin's The Thinker (shown at the top right of this page). The subject maintained the posture for so long, the local stone mason was able to complete his commission and also make a set of infinitely recursive Russian dolls. Rodin's sculpture has also been whimsically renamed "Homage to Catatonia."
Practically, philosophy is useful while trying to disprove atheism, the laws of society, any concept of ethics you might have, even the existence of that chair you're sitting on (although never convincingly enough so as to make you feel that you have to stand up). Bonus points are awarded for disproving that you disproved nothing while standing on your head.
Over the years, philosophy has generally avoided adopting either a purpose or a method. Therefore, it is immune to criticism, because you can never point out that it failed to reach its goal, or that its approach is flawed. If you are unwise enough to try to criticize it anyway, your statements will simply become another branch of philosophy.