Locke followed the Port-Royal Logique (1662) in numbering among the abuses of language those that he calls "affected obscurity" in chapter 10.
Locke complains that such obscurity is caused by, for example, philosophers who, to confuse their readers, invoke old terms and give them unexpected meanings or who construct new terms without clearly defining their intent.
In terms of qualities, Locke divides them into primary and secondary, in which primary give our minds ideas based on sensation and actual experience.
On the other hand, secondary qualities allow our minds to understand something based on reflection, in which we associate what we perceive with other ideas of our own.
Locke writes at the beginning of the fourth chapter, Of the Reality of Knowledge): "I doubt not my Reader by this Time may be apt to think that I have been all this while only building a Castle in the Air; and be ready to say to me, To what purpose all of this stir?
Knowledge, say you, is only the Perception of the Agreement or Disagreement of our own Ideas: but who knows what those Ideas may be? But of what use is all this fine Knowledge of Man's own Imaginations, to a Man that enquires after the reality of things?One of Locke's fundamental arguments against innate ideas is the very fact that there is no truth to which all people attest.He took the time to argue against a number of propositions that rationalists offer as universally accepted truth, for instance the principle of identity, pointing out that at the very least children and idiots are often unaware of these propositions.He also criticizes the use of words which are not linked to clear ideas, and to those who change the criteria or meaning underlying a term.Thus he uses a discussion of language to demonstrate sloppy thinking.Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics, moral philosophy, natural philosophy ("science"), faith, and opinion.Book I of the Essay is devoted to an attack on nativism or the doctrine of innate ideas; Locke indeed sought to rebut a prevalent view, of innate ideas, that was vehemently held by philosophers of his time.Substances are “nothing but the assumption of an unknown support for a group of qualities that produce simple ideas in us”.Despite his explanation, the existence of substances is still questionable as they cannot necessarily be “perceived” by themselves and can only be sensed through the qualities.In anticipating a counter-argument, namely the use of reason to comprehend already existent innate ideas, Locke states, "by this means there will be no Difference between the Maxims of the Mathematicians, and Theorems they deduce from them: All must equally allow’d innate, they being all Discoveries made by the use of reason." Whereas Book I is intended to reject the doctrine of innate ideas proposed by Descartes and the rationalists, Book II explains that every idea is derived from experience either by sensation – direct sensory information – or reflection – "the perception of the operations of our own mind within us, as it is employed about the ideas it has got".In Book II, Locke focuses on the ideas of “substances” and “qualities”, in which substances are “an unknown support of qualities” and qualities have the “power to produce ideas in our mind”.