Since simple ideas are passively received in the mind...

Since simple ideas are passively received in the mind, for example, they must be caused by something real, even though-as we've already seen-in the case of secondary qualities they fail to resemble their causes. [Essay II xxx 1-2] Because our complex ideas of modes and relations are not supposed to refer to anything else beyond their own archetypal content, they are all real as well.

Ideas of substances, however, are intended to refer to existing things and are therefore fantastical if things with the appropriate combinations of features do not in fact exist. [Essay II xxx 4-5] An idea is further said to be adequate only if it represents its intended object fully and perfectly; inadequate ideas convey the nature of their objects only partially.

[Essay II xxxi 1] Locke insisted that all simple ideas are adequate, though doing so required some fancy footwork with respect to our ideas of secondary qualities. Properly understood, he argued, simple ideas represent whatever power it is that produces them in us, whether or not the idea resembles that power-which, in the case of secondary qualities, it does not.

As with the reality of ideas, so with their adequacy the vital point for Locke was the causal process by means of which we acquire them; our lack of voluntary control over that process forestalls any possibility of mistake or erroneous judgment. [Essay II xxxi 2, 12] Because complex ideas of modes and relations are assembled by the mind without reference to any external archetype, they are their own archetypes, which they cannot fail to capture adequately.

Although our communication with each other about such ideas (especially in the case of mixed modes) may falter because we do not agree about the signification of words we both employ, the ideas themselves are invariably adequate. [Essay II xxxi 3-5, 14] Once again, it is complex ideas of substances that are unreliable; such ideas, according to Locke, have a double intended reference but are inadequate in both respects.

If they are supposed to represent the substantial forms of existing things, our ideas of substances are inadequate because (on the corpuscularian theory) these real essences are unknowable.