Change is one of the most evident facts of experience. And yet, it is one of the most formidable metaphysical problems there is. For change seems to require the impossible: that a thing be and not be, under the same respect.

It is precisely for this reason that philosophy, very early on, tried either to deny it, dissolve it, or circumvent it. Let us therefore go back, before conceptual refinements, and look at how thought has collided with this problem—and why only one answer truly holds.

Parmenides: the radical refusal of change

Parmenides starts from an implacable logical intuition:

What is, is; what is not, is not.

So far, nothing to object. The principle of non-contradiction is stated with exemplary rigor. But Parmenides draws a radical conclusion: change is impossible.

Why? Because to change would suppose that a thing passes from what it is not to what it is—in other words, that being springs from non-being. But this is contradictory. Therefore change is only an illusion. Reality is one, immobile, eternal, without becoming.

The position is coherent... but it is unlivable. It denies what all experience manifests, and it negates itself performatively: to reason, conclude, argue, already implies a passage, a before and an after, an intellectual act that was not and now is.

Parmenides is right on a fundamental point: non-being can produce nothing. He is wrong on the conclusion: it is not change that is impossible, it is the way he conceives it.

Heraclitus: the dissolution of change into flux

Heraclitus takes the opposite position. Change is evident, omnipresent, total. Everything flows. Nothing remains. One never steps into the same river twice.

But in wanting to save change at all costs, Heraclitus sacrifices being itself. If everything changes absolutely, then nothing is really identical to itself. There is no longer a subject of change, only an indeterminate flux.

Yet change without a subject is no longer change: it is perpetual disappearance. One does not change something; one simply has a succession of snapshots without ontological continuity.

The Heraclitean position saves immediate experience but destroys intelligibility. One can no longer explain, know, or even identify anything.

The atomists: displacing the problem

The atomists—Democritus foremost—attempt a mediation: change is not real at the fundamental level; it is only a rearrangement of immutable elements.

Atoms do not change. They move, combine, separate. Becoming is saved at the price of an ultimate ontological immobility.

But this solution resolves nothing in depth. It displaces the problem without solving it:

• why do atoms move?
• what is movement, if not already a change?
• how would a mere spatial arrangement explain the appearance of new real determinations?

Moreover, this position already presupposes what it claims to explain: movement, causality, structure. It only pushes the difficulty to a smaller level.

The only real alternative: the realist answer

The realist answer—formulated decisively by Aristotle—consists in taking seriously both the principle of non-contradiction and the reality of change, without sacrificing one to the other.

It rests on a fundamental distinction:

the distinction between act and potency.

To change does not mean to pass from non-being to being. To change means to pass from potency to act.

What changes is not absolutely what it is not; it is capable of being otherwise. This capacity is not nothingness, but a real mode of being.

Thus:

• the acorn is really an oak in potency,
• the child is really an adult in potency,
• the green leaf is really yellow in potency.

Change becomes intelligible without contradiction: the thing remains the same as to its being, while receiving a new determination.

What this answer imposes

This solution is not free. It imposes several heavy but unavoidable consequences:

1. Reality is structured: it is not an undifferentiated block, but composed of distinct principles.

2. Non-being does not act: change never comes from nothing.

3. Becoming is real: it is neither illusion nor mere appearance.

4. Causality is necessary: passing from potency to act presupposes an actualizing principle.

5. Whatever changes is not fully in act: it depends on something else to become what it becomes.

At this stage, a decisive consequence appears, which can no longer be evaded for long:

What is in potency cannot actualize itself.

And with this single sentence, an entire metaphysics is already engaged.