Philosophy isn't a homogoneous discipline: there are philosophers who understood their work as empiricism, others who take it to something pure and formal; some even argue that philosophy is both: for instance, Deleuze sometimes describes his position as "transcendental empiricism."

Given these differences in approach, sometimes even among thinkers who could be described as friendly, and whose work could be described as working through related questions, it is difficult to meaningfully answer this question as posed. Any philosopher worthy of the name will have serious doubts and pointed questions for those claiming the truth can be found only empirically, or only formally.