The main reason why mechanics was a very good candidate for the title ‘Theory of Everything’ is probably that it feels so definitive, not only in its strict and transparent mathematical structure, but also in its workings. If we want to know the future, all we have to do is specify the positions and velocities of all material at some initial time, and then the remainder of eternity can be computed, at least in theory. With a theory of everything, we can say with Leibniz: let's calculate! Calculemus! Voltaire wrote:

Einstein's relativity theories did not change that; again, at least in principle. The computations become vastly more complicated, but the fact remains that, in relativistic mechanics, the future follows uniquely from the past.

Everyday mechanical events usually show that this is an acceptable point of view. In practice, the fact that the Universe consists of an unruly number of particles makes that initial specification and its subsequent computation difficult, but not necessarily impossible (plus the problem that the computer must be part of what it computes, but never mind that).

However, there is a danger lurking under this seemingly calm surface. Because mechanics demands that we specify the whereabouts of all material in the Universe at an initial time, the question arises what that ‘material’ is and, in particular, what its internal constitution is.

It had long been clear that gravity cannot be the only force in the Universe, because gravity is a purely attracting force, so it cannot produce stable objects by itself. But research into electricity had shown that an electric charge can be positive as well as negative, and that charges with the same sign repel each other. The experiments and theories of Faraday and Maxwell, developed in the 19th century, showed that the forces of electricity and magnetism can be in equilibrium. Therefore, electromagnetic forces were good candidates for the explanation of the structure of matter.

Then, in the first quarter of the 20th century, it was discovered that matter is not continuous, but built out of discrete particles. These particles form families, analogous to the well-established ‘chemical elements’, and within a given family the particles are precisely identical. Faraday's electric current turned out to be a stream of an immense number of electrons.