Summary: Monadology

Leibniz defines the monad as a simple substance without parts. The existence of compound bodies demonstrates the existence of monads, since the existence of the compound presupposes the existence of the simple. As a result, these are the true atoms of nature ¹.

What, then, are these absolutely simple entities, the monads?

Indeed, they possess neither extension nor shape. Since extension is divisible, extended bodies are not absolutely simple. Similarly, shapes are divisible—for instance, a triangle can be cut in two—and can only characterise complex bodies.

Likewise, monads can only appear or disappear all at once—through creation or annihilation—for a simple substance has nothing to dissolve. Only compound bodies can have their parts disjoined, just as only composite structures can be formed by the addition of one part to another.

No external movement can affect a monad (again, because of their simplicity, as movement typically involves a change in the arrangement of parts). Consequently, nothing can enter a monad. This leads to the famous assertion:

Monads have no windows, through which anything can enter or leave them ².

At this stage, the monad still appears as something indeterminate—not even as a kind of void, since even a void occupies a certain extension.

Leibniz will then seek to give content to the monad without contradicting its simplicity.

It is necessary, first of all, that monads have some qualities; otherwise, they would not even be beings ³.

Monads must also be distinguishable from one another. Here, Leibniz refers to his principle of identity of indiscernibles, stated in his New Essays on Human Understanding and derived from the principle of sufficient reason, according to which there are never in nature two beings which are perfectly one like the other ⁴.

Monads have no parts, yet they do possess qualities.

While no external movement can affect a monad, it does experience internal movements, like all created beings, arising from an internal principle.

Finally, within the monad there is a plurality of affections and relations, though there are no parts of them ⁵. These are what Leibniz calls perceptions.

The monad is therefore a soul. The simple substance underlying all bodies is, in essence, a soul. Indeed, perception cannot be explained by physical or mechanical bodies alone. This is the essence of the famous passage:

Imagine a machine constructed in such a way as to think, feel, and perceive. Now, suppose it were enlarged to a scale that allowed us to step inside it, like entering a mill. Upon inspecting its interior, we would find only parts pushing against one another—never anything to explain a perception ⁶.

These monads, or souls, may also be called entelechies, owing to the fact that they have a certain perfection, inasmuch as they are themselves the sources of their internal actions. (The term 'entelechy' comes from Aristotle and refers to a being that has reached its end, thereby attaining a certain degree of perfection.)

Leibniz emphasises the importance of memory, which organises our perceptions—something we share with animals. For instance, a beaten dog will flee at the sight of the stick with which it has been struck in the past.

The human being is distinguished from animals by their knowledge of the eternal and necessary truths of Reason.

Two great principles guide our reasoning: that of contradiction (which states that what contradicts the false is true) and that of sufficient reason: nothing happens without a sufficient reason—in other words, there is a reason for everything.

¹ Monadology, 3
² 7
³ 8
⁴ 9
⁵ 13
⁶ 17