But where is the fourth?

I try to turn many but not all threefolds into fourfolds.

The Four Causes meet special relativity. Hilarity does not ensue.

Notice E−pc at the end of the Special Relativity row above.
SPEED stands to SLOWNESS, as net momentum p stands to E−pc (or equivalently E⁄c−p or E⁄c²−p⁄c).
But one can't call E−pc or its equivalents "slowmentum" which already refers to something else. Anyway, I noticed decades ago that E−pc would be decidedly more useful to consider for a system at very high speed (6⁄7 or more of lightspeed) than for one at rest or at least relativistically slow, so to speak (less than 1⁄7 of lightspeed). It doesn't take Einstein to see this. In a rest frame, the magnitude of E−pc is simply equivalent to the rest mass (rest energy). This is likewise as linear energy is more useful to consider for a system at low speeds than for one at lightspeed, where linear energy is equivalent to magnitude of momentum. It's almost a mirror-image kind of thing. It's right there in the arithmetic. Anyway, I've found it said by Google AI (on 2026-2-17 or so) that E−pc is a formula for light-cone momentum and is useful to consider at high speeds. In doodles I used to label it lusk and symbolize it k. I'm not a physicist or a mathematician, but I was fiddling around with these things in search of Aristotle's Four Causes or the like. I think I found them and it's not something lame.

Assume relativistic units (i.e., lightspeed c = 1).
p = net momentum.
e = linear energy.
E = total energy.
m = rest mass (often symbolized by "m₀" as I do earlier in this post).
k = gross minus net momentum, corresponding to internal spatial structure but not invariant in magnitude like rest mass across all rest frames soever varied.

Equations:

Remember, set lightspeed c equal to 1 (to temporarily streamline it out of the equations, jewel though it is.
Relativistic slowness in a direction
= 1−velocity v as a fraction of lightspeed.
k = E(1−v) = E−p.
E² = p² + m².
E = p+k = m+e.
p−m = e−k.
p−e = m−k = p+m−E = E−e−k = √(2ke).
Example:

(Continued from table.) The formed entelechy as cause refers to checks, stable balances, stable structure, standing finished, evidences. The object's formed entelechy in operation for a further end was called second entelechy. I think of concrete form as structure in the sense of spatial or spacelike balance, balancement, of movements or forces (a cloud of differently traveling photons has balance, equilibrium, in its inertial reference frame but little stability). As the good has the rational character of a culminal end, so the true, the sound, has the rational character of an entelechy. An entelechy is a stable establishment or confirmation that the sought good was indeed achieved and was indeed good.)

Aristotle's Four Causes' principles favor special relativity over Newtonian mechanics.

The above distinction between bearer and borne works only in Einsteinian physics, not in Newtonian physics, where the bottom two corners are flatly conflated (always m=k and bearer = borne, so there's no occasion at all to extract and label k and borne) since Newtonian physics recognizes no signal-speed limit such as lightspeed universal to events and invariant across all rest frames, and so recognizes no non-arbitrary quantity of slowness and does not distinguish among rest energy, total energy, and total energy minus (magnitude of) net momentum (E−pc).

Losing one's translational innocence and some incompetence

I used not to know that Ancient Latin patiens served to translate Ancient Greek páschonta (undergoer, sufferer, e.g., "which is cauterized"); páschonta had connotations like those of passion rather than of patience which are nearly the opposite. Instead, I assimilated agent:patient to make:let and must:can. So I felt I had to make a fourfold out of the four causes' traditional three (as Latinized) causal principles: agent, patient, act. Actually, two Ancient Greek words, distinct in form and meaning, came to be habitually translated as actus. I didn't realize what a tangle of Ancient Greek and Scholastic Latin translations there are to be dealt with. Anyway, now I think of the 4 causes from three standpoints. The things that I call causal principles are somewhat the 4 causes but seen not strictly from said process standpoint, more like from a kinetics or mechanics standpoint. Then there is the process standpoint, what I call the Four Causes least loosely. Then there is an enhanced set of 4 causes reflecting classes of phenomena from lower to higher.

Principles of the causes:
A1) Kinoûn, mover. A2) Páschon, patient, bearer, tolerator (internal). A3) Enérgeia, activity; or a thing as affected. A4). Borneness, bornement; or a thing as borne, balanced (internally).

The Four Causes as process:
B1) Inception. B2) Middle, means. B3) Culmination. B4) Entelechy, a kind of refrenation (a refraining or making refrain) or abstention (call it "abstinuation"?), and, howsoever labeled, a staying (standing, sitting, lying, drifting) finished.

The Four Causes as reflecting levels of phenomena:
C1) Motion, force. C2) Matter. C3) Life. C4) Mind.
To regard the C series as the Four Causes leads to holding that a culminal action needs to be a biological function in order to be deemed a final cause, and I think that most people do believe that function explains biological phenomena in way that is, at the least, much clearer than in the case of entirely nonliving things. The C series also leads one to hold that a form or structure needs to be mental or intelligent (an essence, a definiiton, as opposed to an actuality?) in order to be deemed a full-fledged cause. Or instead of the C series, one could stick with the B series and argue for example that thermodynamic decay is the final cause, the explanatory culminal action, of a closed material system outside of a biological context. Or one could assemble these alternate conceptions of cause into a system, for whatever that would be worth. Come to think of it, that's what I've started to do in this paragraph.

Motion involves the idea of analysis into various combinations of motions, as in the case of waves, and the idea of rules and extremal constraints. Matter involves the idea of clumpy ingredients and probabilistic distributions.

The motion-matter-life-mind fourfold also reflects the fourfold of rudimentary conjoined logical quantities in the bottom row of the table at the top of this post. Logical quantity, in the philosophers' sandbox of 1st-order logic with identity, ain't stupid but it's a comic strip next to brain science, and philosophers ought to explore logical quantity one of these centuries.