Some may find it annoying, but one of my favorite things about the English language is that it's full of synonyms that point to roughly the same place in concept space but with each word being a distinct vector carrying nuanced differences in meaning. We see this with "react" and "respond", which both roughly have a meaning of "do something because something else happened", but each of which carries its own implications about the how the doing is done.

"Respond" is the older word so we'll start with it. It comes from Latin through French and is constructed from the roots "re-" and "spondere". "Re-" serves like "back" in its prepositional sense in English as in "go back there" or "do it back to him" and is synonymous with "in return". "Spondere" means "to pledge", but over time "respond" morphed to mean something less like "make a pledge back" and more like "answer back" or "answer in return". The result is our modern word "respond" that connotes a deliberate action made in return to another one, carrying some but not all of the original weight of "pledge" with it.

"React" is newer. It first appears in the 1640s in the sense of a physical reaction from "re-" plus "act" from Latin "actus" which itself draws from Proto-Indo-European "*ag-" which covers the notions of driving, drawing out or forth, and moving. So we can say "react" is to "move in return" to another movement, and it carries that sense through today in a generalized way as both the word we use to talk about what chemicals do when they are "moved" to react and the immediate return actions people make when acted upon.

Together "react" and "respond" create a dimension along we can describe what is done in return. Specifically, we more call those things "reactions" that are done quickly, automatically, or without deliberation and more call those things "responses" that are done carefully, voluntarily, or with deliberation. To give an example, suppose you go to see a movie about factory farming. While watching it you react with disgust, horror, indifference, etc. and may even post about your feelings on social media. After you think about what you saw, though, you may respond to it by changing your behavior or talking about why you think it was distasteful content to show. In this way "react" and "respond" roughly correspond to System 1 and System 2 generating action in return to experiences, respectively.

But that's just a first gloss because I think the S1/S2 distinction is confounded and something more is going. When "react" and "respond" are construed as I have construed them above, most people (especially the sort of people who read LW) would say they would prefer to "respond" to "react". That is, they would prefer to think before choosing an action rather than autonomically acting. Yet, no matter what we do, we will have some autonomic reaction, even if it is the "null" reaction of disregard, so even if we want to wait to respond we must react in some way first. Thus we get at one of the core challenges in rationality: how to react so that we may respond.

Since we already have mathematical models that mostly tell us how to respond if we have enough resources, even if we rarely or never have access to those resources and so must respond in suboptimal ways, it's tempting to create a space where reactions don't count and only responses matter. But to me that seems a dream because for the time being we are all humans and we all react to others reactions, so even if we try to suppress reactions we cannot escape them. Thus it seems we have no choice but to react, and if we must react, it seems to me we might as well react in ways that accord with how we would respond.

So as much as rationality feels like a System 2 or far construal mode activity, it ultimately makes demands on System 1 and near construal mode thinking. As such we must engage with gnosis or personal experience of rationality if we want our reactions to align with our responses and to do the things that actually win rather than merely try. It seems only in this way that we may learn to react so that we may respond.