An idolon (or eidolon) specus is a form of prejudice, by which someone inappropriately extends norms or tenets that apply to his own culture and social group, or to his own preferences. Racism, sexism and, more generally chauvinism are examples of idola specus, but the concept goes beyond them to the criticism of all forms of irreflexive subjectivity or individual predisposition.

The term is Latin, meaning "idol of the den", although idolon is in itself a Latin borrowing from a Greek word meaning image. The alternate spelling eidolon preserves the Greek root more closely. It was introduced by Sir Francis Bacon in his Novum Organon , a treatise on logic and scientific method, to name one of the dangers that the philosopher faces in the process of thought. These dangers or idols are ingrained notions that challenge his ability to differ from established notions. Besides idola specus, there are also idola tribus, idola fori and idola theatri .

Critical Theory, for example, claims that positivist insistence in theoretical consistence, regardless of the possible lack thereof in the thing itself — such as in the analysis of society, which may very well be inconsistent of even irrational in its workings — is derived of a purely conventional impulse towards coherence and uniformity, making it an idolon specus.