Complex ideas, which are very numerous, may be divided into three classes:  Modes, Substances, and Relations.

Modes (states, conditions) are such combinations of simple ideas which do not “contain in them the supposition of subsisting by themselves, but are considered as dependencies on, or affections of substances.”  They fall into two classes according as they are composed of the same simple ideas, or simple ideas of various kinds; the former are called simple, the latter mixed, modes.  Under the former class belong, for example, a dozen or a score, the idea of which is composed of simple units; under the latter, running, fighting, obstinacy, printing, theft, parricide.  The formation of mixed modes is greatly influenced by national customs.  Very complicated transactions (sacrilege, triumph, ostracism), if often considered and discussed, receive for the sake of brevity comprehensive names, which cannot be rendered by a single expression in the language of other nations among whom the custom in question is not found.  The elements most frequently employed in the formation of mixed modes are ideas of the two fundamental activities, thinking and motion, together with power, which is their source.  Locke discusses simple modes in more detail, especially those derived from the ideas of space, time, unity, and power.  Modifications of space are distance, figure, place, length; since any length or measure of space can be repeated to infinity, we reach the idea of immensity.  As modes of time are enumerated succession (which we perceive and measure only by the flow of our ideas), duration, and lengths or measures of duration, the endless repetition of which yields the idea of eternity.  From unity are developed the modes of numbers, and from the unlimitedness of these the idea of infinity.  No idea, however, is richer in modes than the idea of power.  A distinction must be made between active power and passive power, or mere receptivity.  While bodies are not capable of originating motion, but only of communicating motion received, we notice in ourselves, as spiritual beings, the capacity of originating actions and motions.  The body possesses only the passive power of being moved, the mind the active power of producing motion.  This latter is termed “will.”  Here Locke discusses at length the freedom of the will, but not with entire clearness and freedom from contradictions (cf. below).

Modes are conditions which do not subsist of themselves, but have need of a basis or support; they are not conceivable apart from a thing whose properties or states they are.  We notice that certain qualities always appear together, and habitually refer them to a substratum as the ground of their unity; in which they subsist or from which they proceed. Substance denotes this self-existent “we know not what,” which has or bears the attributes in itself, and which arouses the ideas of them in us.  It is the combination