In All Quiet on the Western Front, Erich Maria Remarque describes a battlefield reflex that is “far quicker, much more sure, less fallible, than consciousness”:

A man is walking along without thought or heed;–suddenly he throws himself down on the ground and a storm of fragments flies harmlessly over him;–yet he cannot remember either to have heard the shell coming or to have thought of flinging himself down. But had he not abandoned himself to the impulse he would now be a heap of mangled flesh. It is this other, this second sight in us, that has thrown us to the ground and saved us, without our knowing how.

Apparently this was a real phenomenon, at least in World War I. Stephen Foot, a British officer who spent three years on the western front, writes in his 1934 memoir Three Lives:

It was about this time that I suddenly realised that I had become acclimatised to shell fire; not that I liked or had ceased to fear it, but that I was able to discriminate between a shell that was coming near and one that was far enough away not to worry about. Some officers recently arrived from England were responsible. They were in charge of one of the working parties, and while we were standing in a group, not far from Plug Street, we heard the whine of a German shell. I looked instinctively in the direction of the church to see the usual cloud of dust and stones, and at the same time I realised that the officers with me had ducked. I was certainly no braver than they were — quite the contrary; but my ear had somehow become attuned, so that as soon as the first sound was heard I knew approximately where the shell would burst.

Foot calls this sense “a great relief to one’s nervous system” and speculates that the worst cases of nervous breakdown occurred in men who didn’t have it. “One cannot explain it,” Remarque wrote. “If it were not so, there would not be one man alive from Flanders to the Vosges.”