A Philosophy paper makes a clear, concise, simple statement that you must defend. This statement, also known as a thesis, poses the writer's central argument.
If you state that "X is Y," you must also explain why you believe "X is Y" (See tips on effective language.)
"Socrates believes X; however, from the following discussion I..."
"Socrates believes X. The evidence for this claim is..."
It is perfectly acceptable to state one's opinion--a good thesis is one that takes advantage of the author's interests and beliefs--but that opinion should be reinforced with strong textual support. Be careful to choose a thesis that is both engaging and defensible.
(from the U of Richmond)