From Leibniz, Philosophical Essays, 326 (the letters to Clarke):
12. God is not present to things by situation but by essence; his presence is manifest by his immediate operation. The presence of the soul is quite of another nature. To say that it is diffused all over the body is to make it extended and divisible. To say it is, the whole of it, in every part of the body is to make it divisible of itself. To fix it to a point, to diffuse it all over many points, are only abusive expressions, idols of the tribe.Leibniz's complaint is clear enough. He thinks that the scholastic claim that the soul is in every part of the body amounts to saying that the soul is extended and divisible in itself. What is the A-T or A-S philosopher to say in response? First recall the basic hylemorphic theory: a human being is a composite of matter, generally interpreted as a potential principle, and substantial form, generally interpreted as a principle of actuality. Apart from giving actuality and being to the composite, substantial forms also support accidents. A scholastic philosopher would think of extension and divisibility as accidents, probably falling under the category of quantity. Leibniz wants to argue that since the body is divisible the soul must also, by its very nature, be divisible. But a scholastic would deny this inference, and hold instead that for the soul to be in every part of the body means that, as the subject to the accident of quantity, it is accidentally extended and divisible.