In philosophy skepticism is the notion that we cannot know things beyond a shadow of a doubt.
Extreme skepticism contends that we cannot be certain about the truth of any belief, including the belief in skepticism.
Softer forms of skepticism point to specific branches of inquiry or to a method of doubt that attempts to clarify uncertainties, even if imperfectly so.
One can believe and still be a skeptic, as outlined in the following:
The true meaning of the word skepticism has nothing to do with doubt, disbelief, or negativity. Skepticism is the process of applying reason and critical thinking to determine validity. It’s the process of finding a supported conclusion, not the justification of a preconceived conclusion.
It’s thus inaccurate to say “Skeptics don’t believe in ghosts.” Some do. Many skeptics are deeply religious, and are satisfied with the reasoning process that led them there. Skeptics apply critical thinking to different aspects of their lives in their own individual way. Everyone is a skeptic to some degree.¹
The notion of skepticism is, perhaps, traceable to the apparent humility of Socrates (469–399 BCE), as opposed to the use of Socrates as a literary character by Plato to advocate the theory of Forms. However scholars usually say that the first known skeptic is Pyrrho of Elis (365–275 BCE).
The influential Islamic philosopher and psychologist Al-Ghazali (1058-1111) promoted a type of skepticism that some say may have influenced René Descartes‘ method of doubt as found in his Discourse on the Method. Here, Descartes starts off as a skeptic, finds himself in intellectual hot water, so relies on his essentially theological beliefs about the goodness of God to bail himself out.²
Another type of skepticism is geared more toward corrective social practice—namely, professional skepticism. A professional skeptic would be hired to discover and help prosecute frauds, hostile infiltrators and unduly corrupt individuals hiding out under seemingly legitimate covers.³
¹ The reference to this quote also mentions the popular usage that to be a skeptic is to, more often than not, bash certain ideas: https://skeptoid.com/skeptic.php
² Descartes looks into the problem of solipsism, which I don’t think requires a belief in God to reject. The mere uncertainly should be enough. For what if the solipsist is wrong? Can he or she be a truly ethical person with no respect for the (possible) reality of others?