The first category - substance - is the most important in Aristotle's ontology. Substances are, for Aristotle, the fundamental entities. The Categories is a text from Aristotle's Organon that enumerates all the possible kinds of things that can be the subject or the predicate of a proposition. They are "perhaps the single most heavily discussed of all Aristotelian notions". Aristotle agrees with Plato that knowledge is of what is true and that this truth must be justified in a way which shows that it must be true, it is necessarily true. ... Thus it is through the senses that we begin to gain knowledge of the form which makes the substance the particular substance it is.