Ok so that’s what the first law says but what is energy, anyway? We hear a lot about different kinds of “energies” but the simplest definition of energy comes from the work-energy theorem: Energy is what is capable of doing work. What is work? Work is moving mass over distance, like hauling timber or lifting a coffee cup. If we put all that together then we arrive at the statement: “The total capacity to move mass through space in an isolated system is constant.” I think this statement is an adequate description of the first law. But let me clarify a few points. First, the definition of energy as “how much mass can be moved how far” underscores the fact that energy is a conceptual abstraction. The four forces (but I prefer the word interactions) of nature are understood as the fundamental physical “givens” responsible for this total energy. This is just saying that gravity or electromagnetism, which are the interactions relevant to biology, are the things that “cause” the movement in the mechanistic sense. What we refer to as energy is a way of quantifying the potential for motion that exists because of these interactions. What makes “energy” such a grand unifying abstraction is precisely it’s mathematical property of being a constant quantity. Feynman quipped the following about the first law (my emphasis added):
...It states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same.
Two points about this quote: Energy is a numerical quantity. I would restate that point by saying that energy is a mathematical abstraction. It is not “out there in the world” in the way that trees and electromagnetism are “out there.” I’m fond of the assertion that energy is what is, not because energy is a fundamental substratum or essence of “reality” in any sense but rather precisely for the opposite reason that it is a mathematical abstraction which is invariant in time. So that’s the first point--Energy is not a physical thing but a mathematical idea which was “thingified” by language because it was found to have the immensely useful property of never changing no matter how the actual matter under consideration was found to move. The first point is related to the second, which is that the fact of conservation of energy is not a “mechanism.” It is a rule which “mechanics” obeys but it does not tell you how anything happens. So now a brief digression to explain this point.