"Simple perceptions or impressions and ideas as such as admit of no distinction nor separation. The complex are the contrary to these, and my be distinguished into parts."In inquiring as to whether complex ideas always correspond to complex impressions Hume answers "no" (eg, I can have n idea of a city paved with gold, etc), but with simple ideas, the answer is "yes":
"every simple idea has a simple expression, which resembles it; and every simple impression has a corresponding idea."
"neither the ideas of the memory nor imagination … can make their appearance in the mind, unless their correspondent impressions have gone before to prepare the way for them, yet the imagination is not retain'd to the same order and form with the original impressions; while the memory is in a manner ty'd down in that respect, without any power of variation."
The "liberty of the imagination to transpose and change its ideas"
is responsible for the fanciful objects (winged horses, fiery dragons,
etc) we meet in fables. We will not consider this liberty of the imagination
strange "when we consider, that all our ideas are copy'd from our impressions,
and that there are not any two impressions which are perfectly inseparable."
This is a consequence of the distinction between simple and complex ideas.
"Where-ever the imagination perceives a difference among ideas, it can
easily produce a separation." This principle has important implication
for understanding the nature of relations as is brought out in the following
section.
"As all simple ideas may be separated by the imagination, and may be united again in what form it pleases, nothing wou'd be more unaccountable than the operations of that faculty, were it not guided by some universal principles which render it, in some measure, uniform with itself in all times and places. Were ideas entirely loose and uncorrected, chance alone wou'd join them; and 'tis impossible the same simple ideas should fall regularly into complex ones (as they commonly do) without some bond of union among them, some associating quality, b which one idea naturally introduces another. This uniting principle among ideas is not to be consider'd as an inseparable connexion; for that has been already excluded from the imagination: Nor yet are we to conclude, that without it the mind cannot join two ideas; for nothing is more free than that faculty: but we are only to regard it as a gentle force, which commonly prevails… The qualities, from which this association arises, and by which the mind is after this manner convey'd from one idea to another, are three, viz. RESEMBLANCE, CONTIGUITY in time or place, and CAUSE and EFFECT. … These are therefore, the principles of union or cohesion among our simple ideas, and in the imagination supply the place of that inseparable connection, by which they are united in our memory."