Of course, deciding how concepts should be defined—and whether it makes sense to continue using them—has traditionally been the job of philosophers, not scientists. Against the backdrop of these scientific advances, contemporary philosophers have positioned themselves on the border of determinism and free will in an increasingly baroque and clashing array, like a mosaic composed of mutually repelling magnets. Each of these magnets has a very specific designation. There are hard incompatibilists, soft incompatibilists, compatibilists, semicompatibilists, libertarians (not the political sort), hard determinists, impossibilists, illusionists, and on and on. (It’s difficult here not to pull for the so-called daring soft libertarians, if only because their name makes them sound like slightly naughty throw rugs.) For present purposes, it isn’t necessary for us to understand every aspect of these approaches, many of which have the intricacy of Swiss watches. It’s enough to say that nearly all of them can be accommodated by “The Road Not Taken.” Consider the way in which the poem encompasses the following positions, two of which are diametrically opposed:

A Hard Determinist believes that the world is causally determined, and that free will is an illusion. “The Road Not Taken” can easily be read as giving us a speaker who makes the choice he was always going to make, and who will later engage in after-the-fact justification.

Libertarians believe that determinism is incompatible with genuinely free will, that we do have such free will, and that determinism is therefore false. This position is in line with the popular interpretation of the poem as a choice made by an empowered, self-determining speaker who will later know that his decisions made “all the difference” in his own life.

Compatibilists (the majority of contemporary philosophers) think that some variant of determinism is likely true, but that we nonetheless have free will—though a very different sort of free will than people usually assume. A compatibilist might say that the speaker took the road he selected because of his own beliefs and desires, and so his choice was in that sense free.

And so on. When one considers the enormous complexity of the issues surrounding decision making and the sheer amount of disagreement about how and why we choose, it is staggering to see how effortlessly Frost’s seemingly simple poem holds every perspective in equal regard. It’s as if, from the depths of 1915, the poet anticipated not just the arguments that people would make a hundred years later, but the vast shadow that dispute would cast over the culture.