Spinoza and Quantum Field Theory
Quantum field theory does not consider matter to be made out of discrete little balls, and neither did Spinoza.
... matter is everywhere the same, and there are no distinct parts in it except insofar as we conceive matter as modified in various ways. - Spinoza
Spinoza didn't subscribe to the idea that matter is made up of discrete little billiard balls that interact with each other and form the building blocks of the things we see in everyday life. Being an advocate of a modified form of Aristotelian substance, a pervasive "stuff" that everything was made from, he saw matter as a single stuff modified in various ways in all locations in space. A table is made from the same fundamental stuff as a glass bottle; it's just that substance is modified differently in the glass bottle than in the table. In other words, what Spinoza was advocating was a kind of "substance field" - a continuous medium that is modified at every point by the conditions surrounding it. He says this all in proposition 15 in part one of The Ethics:
For example, we conceive water to be divisible and to have separate parts insofar as it is water, but not insofar as it is material substance. In this latter respect it is not capable of separation or division. Furthermore, water, qua water, comes into existence and goes out of existence; but qua substance it does not come into existence nor go out of existence. - Spinoza
The Greek notion of atoms and other particles from which atoms themselves are constituted reigned for much of the early period in quantum mechanics. Unlike classical mechanics, where variables (mass, position, time, momentum) are continuously variable, some of these variables can take only specific values in a particular situation - hence the term "quantum."
We are all familiar with gravitational and electrical fields. You stay firmly planted on the Earth because of a gravitational field, and lightning comes about in an electric field. Classical field theory treats both these fields as continuous to take on any value at any position in space. Obviously, this wasn't going to work with atomic particles that obey the laws of quantum mechanics, where the values associated with position and energy do not take on continuous values.
Quantum field theory (QFT) results from almost a hundred years of thrashing about and playing with very complex mathematics. In essence, it says that atomic particles are not little balls that exist at a single point but are fields that interact. QFT accounts satisfactorily for everything that happens in the world of nuclear particles, and it even embraces Einstein's special relativity, where space and time change according to relative motions.
In essence, QFT signified a change from thinking of matter as made of little balls to thinking of it as a field that spreads through space. This is not the same as Spinoza's claim that all matter is just modified instances of the one substance, but if we are ever to have a theory of everything, then it seems likely that this is the way we will have to go.