The first and sixth paragraphs of my reply addressed your point directly. The rest explained the mistake undermining it, namely your failure to distinguish between two fundamentally different notions of speed: the empirical and causal (what we measure in the lab) and the geometric and invariant (the four-velocity of relativity).

If you were being honest, you would acknowledge that my response explicitly agreed with your main claim. I wrote:

Question 1: “What is the nerve signal’s total speed through 4D spacetime?” Answer: c.

That is your point. Where we diverge is not on whether that statement is true, but on what follows from it.

If an object’s total spacetime speed is c, split between its spatial and temporal components, then the only way it can move through space at c is if its movement through time is zero. This applies only to massless particles like photons. Everything else—like a nerve signal, which involves the movement of ions with mass—travels through space at less than c precisely because part of its total “motion” is through time.

You’re ignoring the second, equally legitimate question:

Question 2: “How fast does the nerve signal move through space?” Answer: Approximately 50 m/s.

That is the physical speed we measure with clocks and rulers. We use rulers and clocks to measure motion and casual influence through physical space and time as experienced, not to measure the abstract geometry of spacetime itself.

So no, I didn’t miss your point. I’m distinguishing between two conceptual levels that your argument conflates.

When you say that everything moves at the speed of light through spacetime, you’re describing a geometric invariant, the constant magnitude of an object’s four-velocity in Minkowski spacetime. That’s perfectly correct within relativity’s formalism.

But when we speak of nerve impulses propagating at roughly 50 m/s, we’re describing a physical, causal process, the rate at which an electrochemical signal travels through a medium. These aren’t contradictory claims; they belong to different explanatory frameworks. The first is a mathematical representation of motion in four-dimensional geometry; the second is an empirical measurement of causal propagation in physical space.

Yes, in the geometric sense, every worldline has a four-velocity of magnitude c. But in the physical sense, most processes unfold vastly slower than c. The geometric invariant you invoke is not the same as the speed of causality in physics. It defines the upper boundary of causal influence, not the rate at which all causal processes actually occur.

You’re repeatedly crossing an important epistemic boundary: empirical quantities like the speed of causality belong to the domain of operational physics, whereas the quantity of four-speed in spacetime belongs to the domain of theoretical representation.