This Latin term was coined by Sir Francis Bacon and used in his Novum Organum, one of the earliest treatises arguing the case for the logic and method of modern science.
[1] The idola specus are prejudices, by which individuals inappropriately extend norms or tenets that derive his or her own culture and social group, or to his or her own preferences.
Racism, sexism and, more generally just "biases" are examples of idola specus, but the concept goes beyond them to the criticism of all forms of irreflexive subjectivity or individual predisposition.
Whence it was well observed by Heraclitus that men look for sciences in their own lesser worlds, and not in the greater or common world.In more detail, Bacon said that there are a "great number and variety" of idols of the cave but he chooses to select examples which give "the most important caution", and which "have most effect in disturbing the clearness of the understanding".
[1] He judged that they "grow for the most part either out of the predominance of a favorite subject, or out of an excessive tendency to compare or to distinguish, or out of partiality for particular ages, or out of the largeness or minuteness of the objects contemplated.