A philosophical skeptic is someone who questions whether we can have certain knowledge about various things—or in more extreme forms, whether we can have knowledge at all.

There are different types and degrees of skepticism:

**Ancient skepticism** (like the Pyrrhonists) involved suspending judgment on most claims because they believed we lack sufficient evidence to know what's true. They thought this suspension of judgment would lead to mental peace.

**Cartesian skepticism** uses doubt as a method—Descartes famously doubted everything he could in order to find what was absolutely certain (he landed on "I think, therefore I am").

**Modern epistemological skepticism** focuses on specific domains. For example, skeptics might question whether we can know about:

- The external world (maybe we're in a simulation or dreaming)

- Other minds (can you really know that other people are conscious?)

- The past (do you truly know your memories are reliable?)

- Moral truths

- Scientific theories

Most philosophers aren't full skeptics—they think skeptical arguments are useful for testing our reasoning and understanding the limits of knowledge, but that total skepticism is either incoherent or impractical. Still, skeptical challenges have shaped much of philosophy, pushing thinkers to better justify their claims and understand what knowledge requires.

The term is different from everyday "skepticism," which usually just means being appropriately doubtful or requiring good evidence—something most philosophers would endorse.