 From these stipulations he derives his most fundamental and famous claim (1) that “the existence of an idea consists in being perceived” (1710: Part I, §2) or that “their esse is percipi” (1710: Part I, §3) by the perceiving, active mind. Already here Berkeley has the means in place to cast into doubt the meaningfulness of the assumption that there might exist unperceived objects or things. This is due to his restriction of existence to what is perceivable or, even narrower, to what is perceived: If the only objects that exist for a mind—whether it is my own mind or the mind of other human beings or the divine mind—are ideas because there is nothing else that can exist for the mind, then the very concept of something that exists but is not for the mind or is unperceived is a contradiction in terms. Thus if, as Berkeley supposes Locke does, one thinks of things as consisting of collections of ideas, he asks how could one take a thing to be something other than ideas and nevertheless to exist? This question underlies Berkeley’s confidence in what is often referred to as his “master argument”, the argument that one cannot conceive of anything existing unconceived because in trying to do so one is still conceiving of the object (1710: Part I, §23). This seems open to the obvious objection that he is confusing the content of a proposition (for example, “The earth may still exist after the extinction of all conscious life”) with the act of entertaining (“conceiving”) such a proposition, which of course cannot take place except in a conscious being; but if he is already committed to the thought that objects of knowledge are nothing but ideas, it is at least understandable that he should overlook this distinction.