I think this is saying that if you throw meaning and ethics out the window, everything becomes a practical statement.
But even a practical statement – in a world of only physics – is still arbitrary and meaningless.
The argument goes that “right or wrong” can have little basis in a world where only stardust exists.
You need a moral lawgiver for more than that.
The retort to this is often: “No, humans do not follow morals and ethics because of some creator giving us moral laws. We follow them because it allows us to cooperate together and survive.”
Lewis is pointing out that this objection is arbitrary and meaningless too.
Not all want to cooperate. And why are those that want to cooperate and survive any more right or wrong than those that don’t? Whay does any of it mean anything? And who are you to say either way? Why is thought A more or less valid than thought B? By what standard?
See it?
For if all values are socially constructed and in relation to Nature arbitrary, then even the claim that some act is necessary for the perpetuation of the human race is also arbitrary. For why should the human race survive?
-Alan Jacobs, The Year Of Our Lord 1943 (Amazon)