Pre-scriptum: For those who do not like to read, I produced a very short YouTube presentation/video on this topic. About 15 minutes – same time as it will take you to read this post, probably. Check it out: https://www.youtube.com/watch?v=sJxAh_uCNjs.
We think of space and time as fundamental categories of the mind. And they are, but only in the sense that the famous Dutch physicist H.A. Lorentz conveyed to us: we do not seem to be able to conceive of any idea in physics without these two notions. However, relativity theory tells us these two concepts are not absolute and we may, therefore, say they cannot be truly fundamental. Only Nature’s constants – the speed of light, or Planck’s quantum of action – are absolute: these constants seem to mix space and time into something that is, apparently, more fundamental.
The speed of light (c) combines the physical dimensions of space and time, and Planck’s quantum of action (h) adds the idea of a force. But time, distance, and force are all relative. Energy (force over a distance), momentum (force times time) are, therefore, also relative. In contrast, the speed of light, and Planck’s quantum of action, are absolute. So we should think of distance, and of time, as some kind of projection of a deeper reality: the reality of light or – in case of Planck’s quantum of action – the reality of an electron or a proton. In contrast, time, distance, force, energy, momentum and whatever other concept we would derive from them exist in our mind only.
We should add another point here. To imagine the reality of an electron or a proton (or the idea of an elementary particle, you might say), we need an additional concept: the concept of charge. The elementary charge (e) is, effectively, a third idea (or category of the mind, one might say) without which we cannot imagine Nature. The ideas of charge and force are, of course, closely related: a force acts on a charge, and a charge is that upon which a force is acting. So we cannot think of charge without thinking of force, and vice versa. But, as mentioned above, the concept of force is relative: it incorporates the idea of time and distance (a force is that what accelerates a charge). In contrast, the idea of the elementary charge is absolute again: it does not depend on our frame of reference.
So we have three fundamental concepts: (1) velocity (or motion, you might say: a ratio of distance and time); (2) (physical) action (force times distance times time); and (3) charge. We measure them in three fundamental units: c, h, and e. Che. 🙂 So that’s reality, then: all of the metaphysics of physics are here. In three letters. We need three concepts: three things that we think of as being real, somehow. Real in the sense that we do not think they exist in our mind only. Light is real, and elementary particles are equally real. All other concepts exist in our mind only.
So were Kant’s ideas about space and time wrong? Maybe. Maybe not. If they are wrong, then that’s quite OK: Immanuel Kant lived in the 18th century, and had not ventured much beyond the place where he was born. Less exciting times. I think he was basically right in saying that space and time exist in our mind only. But he had no answer(s) to the question as to what is real: if some things exist in our mind only, something must exist in what is not our mind, right? So that is what we refer to as reality then: that which does not exist in our mind only.
Modern physics has the answers. The philosophy curriculum at universities should, therefore, adapt to modern times: Maxwell first derived the (absolute) speed of light in 1862, and Einstein published the (special) theory of relativity back in 1905. Hence, philosophers are 100-150 years behind the curve. They are probably even behind the general public. Philosophers should learn about modern physics as part of their studies so they can (also) think about real things rather than mental constructs only.