The first law of thermodynamics could also be called the first law of physics, or the first law of science. It is simply the common sense rule that in the observable world, nothing can be totally created or destroyed. Most scientists and laypeople would take this as one of the most basic facts about the objective world: that what exists will continue to exist. And that things can not come to existence out of nothing.

So why is this concept a law? Why does our intuitive sense of object permanence apply to the world? There are three ways that this could become a scientific law:

1. As a matter of logical necessity: this seems the most likely, since it seems to follow from the basic laws of mathematics and arithmetic, that an equation must balance. Three and four equal seven. Three particles and four particles equal seven particles. It seems to make sense, but it relies on the assumption that the external world's processes follow mathematical logic. Once we make that assumption, other consequences follow from it logically, but it itself is not a logical assertion.
2. From experimental evidence: This would seem to fit, in that most common experiences, and most scientific experiments, end up with the same total amount of energy and matter as you started with. This is true when making spaghetti, and it is true in particle physics. But there are non-trivial questions about this assumption. For one thing, measurements are not always accurate, even in the example of a pot of spaghetti. While it might be proven that 99.999999999% of the mass and energy is still there, it does not prove the law. This is even more of a concern in the case of atomic physics, where the mass and energy if nuclear processes can be carried away in the form of neutrinos, particles that are all but undetectable. Although a few neutrinos can be detected, there is no experimental way to determine whether mass is being "converted into neutrinos" or whether mass is "disappearing". This could also be the case with very low energy photons, as well as with other particles.
3. As a metaphysical concept. This means that we can conceptualize entities, and conceptualize them as being made of an underlying substance that is neither created nor destroyed. And it is not so much that we are conceptualizing this, as that we can't conceptualize things otherwise. In fact, it is very difficult to even verbally phrase what disappearance is: if we say that something "goes away", we are still verbally postulating its continued existence, perhaps just by force of habit. It still exists, it is just "away". Total disappearance is a hard thing for the mind to picture. In this way, the First law of thermodynamics is less a scientific concept as it is a Kantian category: a basic way of looking at the universe that seems to be built into our mind.

Therefore, the first law of thermodynamics, rather than being a law of science, is a law that stands about science, and shapes the ways that we can possibly view the physical world.