There are different disciplines within the field of philosophy. The scientifically-minded philosopher might be interested in Metaphysics & Epistomology, or roughly, "what we know and how we know it". A standard approach in these fields is to address the Skeptic; someone who logically disagrees with the fundamental assumptions of the philosopher. This is where you get someone like Descartes writing "I think therefore I am" as a starting point for further arguments.

Other disciplines are less concerned with Truth (big "T") so much as they're focused on practical lessons on how to live life.