usual order of nature.

But, when any consistent train of sensitive or voluntary ideas is flowing
along, if any external stimulus affects us so violently, as to intrude
irritative ideas forcibly into the mind, it disunites the former train of
ideas, and we are affected with surprise. These stimuli of unusual energy
or novelty not only disunite our common trains of ideas, but the trains of
muscular motions also, which have not been long established by habit, and
disturb those that have. Some people become motionless by great surprise,
the fits of hiccup and or ague have been often removed by it, and it even
affects the movements of the heart, and arteries; but in our sleep, all
external stimuli are excluded, and in consequence no surprise can exist.
See Section XVII. 3. 7.

18. We frequently awake with pleasure from a dream, which has delighted us,
without being able to recollect the transactions of it; unless perhaps at a
distance of time, some analogous idea may introduce afresh this forgotten
train: and in our waking reveries we sometimes in a moment lose the train
of thought, but continue to feel the glow of pleasure, or the depression of
spirits, it occasioned: whilst at other times we can retrace with ease
these histories of our reveries and dreams.

The above explanation of surprise throws light upon this subject. When we
are suddenly awaked by any violent stimulus, the surprise totally disunites
the trains of our sleeping ideas from these of our waking ones; but if we
gradually awake, this does not happen; and we readily unravel the preceding
trains of imagination.

19. There are various degrees of surprise; the more intent we are upon the
train of ideas, which we are employed about, the more violent must be the
stimulus that interrupts them, and the greater is the degree of surprise. I
have observed dogs, who have slept by the fire, and by their obscure
barking and struggling have appeared very intent on their prey, that shewed
great surprise for a few seconds after their awaking by looking eagerly
around them; which they did not do at other times of waking. And an
intelligent friend of mine has remarked, that his lady, who frequently
speaks much and articulately in her sleep, could never recollect her dreams
in the morning, when this happened to her: but that when she did not speak
in her sleep, she could always recollect them.

Hence, when our sensations act so strongly in sleep as to influence the
larger