In philosophy papers, your thesis will state a position or claim.
The thesis is the most important part of your paper; it tells the reader what your stance is on a particular topic and offers reasons for that stance.
Since the rest of your paper will be spent defending your thesis--offering support for the thesis and reasons why criticism of the thesis may not be valid--it's crucial that you develop a strong thesis.
A strong thesis will:
Source: Writing Guide for Philosophy. George Mason University.
Image source: Sergui Bacioiu. Ripple effect on water. CC BY 2.0. Wikimedia Commons.