Two fountains: sensation and reflection

Having emptied the mind in Book I, Locke must now refill it — but only from experience. In Book II he asks: where does the vast store of ideas come from? “To this I answer, in one word, from experience.” And experience has exactly two sources, two fountains from which all our ideas flow (Essay II.i).

Fountain one: SENSATION (outer sense). Our senses, affected by external objects, convey into the mind ideas of sensible qualities — the yellow of gold, the cold and hardness of ice, the sound of a bell, the smell of a rose. This is the great source of most of the ideas we have, and it depends “wholly upon our senses” turned outward toward the world (Essay II.i.3).

Fountain two: REFLECTION (inner sense). The other source is the mind’s perception of its own operations as it goes to work on the ideas it has. When we notice ourselves perceiving, thinking, doubting, believing, reasoning, willing — this inward observation furnishes a whole second set of ideas, ones the outer senses could never supply. Locke calls it “internal sense” (Essay II.i.4). Sensation looks out; reflection looks in.

Now Locke sorts all ideas along a second axis: simple versus complex. This distinction, cutting across sensation/reflection, is the real engine of Book II. It explains how two humble fountains can yield the mind’s endless variety.

Simple ideas are received passively. Each is “one uniform appearance, or conception in the mind, not distinguishable into different ideas” (Essay II.ii.1) — the plain redness of this rose, the single note of that bell, the warmth of the fire. They are the atoms of thought: the smallest, unmixed units, coming in through sensation or reflection just as the world (or the mind’s own working) presents them. In receiving them the understanding is “merely passive” — it cannot refuse them or alter them.

A crucial consequence: the mind can neither make nor destroy a simple idea. It is wholly beyond the power of the sharpest intellect to invent a new simple idea not taken in by sensation or reflection, or to blot one out. You cannot reason your way to a brand-new primary flavour or colour; you can only receive them.

Complex ideas are where the mind turns active. Out of the stock of simple ideas the understanding combines, compares, and abstracts — building larger structures the senses never handed over whole. Here the mind is a maker, not a mere receiver. Yet — and this is the whole point — it can only ever recombine the simple atoms it was given. It builds new houses, but every brick came from the two fountains.

Locke sorts complex ideas into three kinds (Essay II.xii): modes — ideas that don’t subsist on their own but are affections of substances (a triangle, gratitude, a murder, the number seven); substances — combinations taken to represent distinct things existing by themselves (gold, a horse, a man); and relations — ideas got by comparing one idea with another (father, taller than, cause and effect). All three are assembled from simple ideas.