 That PQs and SQs are placed in the same category, that of things that subsist in substance, reflects Locke’s interest in what we know by means of the senses. For him, substance and quality are inseparble. So if we know that qualities exist, we can rightly infer there is a substance in which they inhere, and we know qualities exist because we receive ideas of sensation caused by substances. Ideas of sensation are one of two sorts of ideas which are the simple elements of all our thoughts and can only be acquired in experience according to Locke. Essay, book 2, chapter 8, which exposits and defends the distinction of PQs and SQs, comes immediately after five chapters that define and classify simple ideas. The sixth breaks down the relation between simple ideas of sensation and qualities in order to show that most of the former do not resemble the later.