Novum organum, as suggested by its name, is focused just as much on a rejection of received doctrine as it is on a forward-looking progression. In Bacon's Idols are found his most critical examination of man-made impediments which mislead the mind's objective reasoning. They appear in previous works but were never fully fleshed out until their formulation in Novum organum:
- Idols of the Tribe (Idola tribus)
“Idols of the Tribe are rooted in human nature itself and in the very tribe or race of men. For people falsely claim that human sense is the measure of things, whereas in fact all perceptions of sense and mind are built to the scale of man and not the universe.” (Aphorism 41.)
Bacon includes in this idol the predilection of the human imagination to presuppose otherwise unsubstantiated regularities in nature. An example might be the common historical astronomical assumption that planets move in perfect circles.
- Idols of the Cave (Idola specus)
These “belong to the particular individual. For everyone has (besides vagaries of human nature in general) his own special cave or den which scatters and discolours the light of nature. Now this comes either of his own unique and singular nature; or his education and association with others, or the books he reads and the several authorities of those whom he cultivates and admires, or the different impressions as they meet in the soul, be the soul possessed and prejudiced, or steady and settled, or the like; so that the human spirit (as it is allotted to particular individuals) is evidently a variable thing, all muddled, and so to speak a creature of chance...” (Aphorism 42).
This type of idol stems from the particular life experiences of the individual. Variable educations can lead the individual to a preference for specific concepts or methods, which then corrupt their subsequent philosophies. Bacon himself gives the example of Aristotle, “who made his natural philosophy a mere slave to his logic”. (Aphorism 54.)
- Idols of the Market (Idola fori)
These are "derived as if from the mutual agreement and association of the human race, which I call Idols of the Market on account of men's commerce and partnerships. For men associate through conversation, but words are applied according to the capacity of ordinary people. Therefore shoddy and inept application of words lays siege to the intellect in wondrous ways” (Aphorism 43).
Bacon considered these "the greatest nuisances of them all" (Aphorism 59). Because humans reason through the use of words they are particularly dangerous, because the received definitions of words, which are often falsely derived, can cause confusion. He outlines two subsets of this kind of idol and provides examples (Aphorism 60).
- First, there are those words which spring from fallacious theories, such as the element of fire or the concept of a first mover. These are easy to dismantle because their inadequacy can be traced back to the fault of their derivation in a faulty theory.
- Second, there are those words that are the result of imprecise abstraction. Earth, for example, is a vague term that may include many different substances the commonality of which is questionable. These terms are often used elliptically, or from a lack of information or definition of the term.
- Idols of the Theatre (Idola theatri)
“Lastly, there are the Idols which have misguided into men's souls from the dogmas of the philosophers and misguided laws of demonstration as well; I call these Idols of the Theatre, for in my eyes the philosophies received and discovered are so many stories made up and acted out stories which have created sham worlds worth of the stage.” (Aphorism 44.)
These idols manifest themselves in the unwise acceptance of certain philosophical dogmas, namely Aristotle's sophistical natural philosophy (named specifically in Aphorism 63) which was corrupted by his passion for logic, and Plato's superstitious philosophy, which relied too heavily on theological principles.