α Causa Per Accidens
As all diseases have two conjunct causes, one immediate, external and exciting, the other constitutional, internal, and predisposing, so it is with Dreams, which are Dis-Eases, or unbalanced States of Consciousness, Disturbers of Sleep as Thoughts are of Life.
This exciting Cause is commonly of two kinds: videlicet, imprimis, the physical Condition of the Sleeper, as a Dream of Water caused by a shower without, or a Dream of Strangulation caused by a Dyspnoea, or a Dream of Lust caused by the seminal Congestions of an unclean Life, or a Dream of falling or flying caused by some unstable Equilibrium of Body.
Secundo, the psychic condition of the Sleeper, the Dream being determined by recent Events in his Life, usually those of the Day previous, and especially such Events as have caused Excitement of Anxiety, the more so if they be unfinished or unfulfilled.
But this exciting Cause is of a superficial Nature, as it were a Cloke or a Mask; and thus it but lendeth Aspect to the other Cause, which lieth in the Nature of the Sleeper himself.
 On Dreams, (a) External Causes