The Platonic doctrine of reminiscence is then adduced as a confirmation of the pre-existence of the soul. Some proofs of this doctrine are demanded. One proof given is the same as that of the Meno, and is derived from the latent knowledge of mathematics, which may be elicited from an unlearned person when a diagram is presented to him. Again, there is a power of association, which from seeing Simmias may remember Cebes, or from seeing a picture of Simmias may remember Simmias. The lyre may recall the player of the lyre, and equal pieces of wood or stone may be associated with the higher notion of absolute equality. But here observe that material equalities fall short of the conception of absolute equality with which they are compared, and which is the measure of them. And the measure or standard must be prior to that which is measured, the idea of equality prior to the visible equals. And if prior to them, then prior also to the perceptions of the senses which recall them, and therefore either given before birth or at birth.
— Plato
Plato is doing something precise here, and it is worth watching carefully before agreeing with it. The argument moves from a genuine observation — that when we see two roughly equal sticks, we immediately measure them against something they fail to match — to a conclusion that pre-loads what soul is and where it belongs. The shortfall is real. Material equals do fall short of equality itself; the soul does seem to bring a standard the senses alone could not have generated. But the direction Plato sends that observation is toward prior existence, toward the soul's original home in a realm above sensory imprecision, toward knowledge as memory of what was once directly seen.
Notice what this structure does to desire. If the soul already knew, and the knowing was lost, then longing becomes the ache of a displaced aristocrat — something originally fine, temporarily confused, capable of recovery through the right philosophical discipline. Suffering, on this reading, is an epistemological problem. The remedy is ascent. The standard preexists the mess; the work is getting back to it. This is not the only thing the shortfall could mean. It might mean the soul is constitutionally oriented toward what it cannot fully hold, that the gap is productive rather than remediable, that the standard was never possessed and the ache is the only honest relationship to it. Plato has chosen one reading of the wound and called it a memory.
Plato·Phaedo