Right, thank you both for your answers! I think Cavacava nailed it. The quotation is what that made me wonder, who/what W is citing and how the notion was originally used.

But I've seen it in other texts, too. It's a notion, a quote, I just don't know where from and what it means...

284. Look at a stone and imagine it having sensations.—One says to oneself: How could one so much as get the idea of ascribing a sensation to a thing? One might as well ascribe it to a number!—And now look at a wriggling fly and at once these difficulties vanish and pain seems able to get a foothold here, where before everything was, so to speak, too smooth for it. And so, too, a corpse seems to us quite inaccessible to pain.—Our attitude to what is alive and to what is dead, is not the same. All our reactions are different.—If anyone says: "That cannot simply come from the fact that a living thing moves about in such-and-such a way and a dead one not", then I want to intimate to him that this is a case of the transition 'from quantity to quality'. (From Wittgenstein's Philosophical Investigations)