Maybe context?

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)

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...

Thank you for a context! I think I can figure it out per the context, but that's just reading, and I'm thinking you can read as well as I can - is that what you have in mind?

Quantity is a question of how much. Quality is either-or. This can be stretched out, but I think this is the essence of it.

If the ability to feel pain is a quality (either-or), and someone has that quality, then it might well be reasonable to ask how much pain they can feel. And of course if they lose that quality and can no longer feel pain, then the question about quantity becomes eo ipso no longer reasonable.

"Transition," I suppose, refers to a change in quality. (E.g., from can feel pain to cannot feel pain.) Or in some cases even a change in quality from a non-zero to a zero value. It looks like just a matter of what you want to call it, here.

Transition, though, is just a general term. To go further you'd have to have some idea of just what the transition was.

If it's a question of how some people react, well, there's no accounting for how some people react.

But this is all simple stuff; I'm pretty sure I've missed what you had in mind, Or is this it?

Trotsky and the molecular structure of the revolution...“tobogganing towards catastrophe,”

All social changes, minute at first , accumulates unseen and unknown by most continuing to form until the quantity of this unrest reaches critical level, the tipping point, and they become a new quality, a new structure a social explosion of a revolutionary quality.

Kaboom!

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.