In Part I of the Principles, Berkeley discusses two ways by which we may come to know an object. The first way (or “head” as he calls it) is through sensory perception, an entirely passive faculty which Berkeley refers to as the way of “ideas” (Berkeley, p.56). The second “head” Berkeley calls the way of “spirits” by which he means an active faculty, apprehending its object either through inference or through immediate (non-sensory) acquaintance.
As far as the first head goes, the story is familiar empiricism but with an idealist twist: the ideas we have in the mind are sensory impressions or “images” copied from things outside the human mind. The twist is twofold: rather than understanding these things external to the human mind to be material bodies, Berkeley understands them to be simply more ideas, but with (i) a higher degree of reality than the images of sense impression or the “chimeras” of imagination and (ii) existence in the mind of God. Since Berkeley refers to all of these things (sensations, human-mind independent objects, and human-mind dependent imaginings) as ideas it will be helpful to map out some distinctions.
Ideas (real) exist in the mind of God and are the originals of which our ideas (images) are mere copies. Ideas (chimeras) are like images but they do not represent (directly) anything real outside the human mind. This distinction in the taxonomy of ideas seems to be relying on the following principle or criterion of “reality”:
An idea has a higher degree of reality the more it satisfies the following conditions
- The idea manifests itself with efficacy, order and distinctness
- The idea must not depend for its existence on a human will (p.36)
- The idea must not depend for its existence on a human mind.
Assuming this reconstruction is accurate, it is interesting to match each type of idea against these criteria. For example, ideas (real) satisfy 1,2 and 3 as they are “affecting, orderly and distinct” (ibid) and do not rely on the human will nor need to be perceived by a human being in order to exist. However, ideas (images) satisfy 1 and 2 but not 3 while ideas (chimeras) seem to satisfy none.
Is there room, I wonder, for a kind of idea that Berkeley has missed? Namely, an idea that is “affecting, orderly and distinct” (1) but that does depend on the human will and must exist in the human mind? For instance, the notion of the “I” seems to fit the bill but for the fact that Berkeley calls it “absurd” that an active substance which “supports or perceives ideas should itself be an idea or like an ideas” (p.78). Perhaps this is an unhappy use of the word “idea” for Berkeley but it would be a convenient way to justify why we might be committed to ascribing a high degree of reality to the concept of the “I” whilst refraining from associating it with the mode of knowledge Berkeley calls the “way of ideas” which he opposes to the “way of spirits”.
 The distinction between ideas (images) and ideas (chimeras) may merely be verbal since in places, Berkeley seems to lump both categories together (e.g. §33 “The ideas imprinted on the senses by the Author of Nature are called real things; and those excited in the imagination being less regular, vivid and constant, are more properly termed ideas, or images of things, which they copy and represent.” (p.35)) Nevertheless, I think there is room for the conceptual distinction since chimeras clearly depend on the human will to a much higher degree than do our sense impressions.
 All in-text paginations refer to Berkeley, George, Treatise Concerning the Principles of Human Knowledge, edited by Kenneth P. Winkler, Hackett Publishing, Cambridge
 I hesitate to use the word “acquaintance” here to talk about the way Berkeley thinks we know, for example, our own mind or God’s. All we need keep in mind is that this word in no way connotes a receptive relationship to the object thus known.