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We have ordinarily as clear (or clearer) Notion of the Relation, as of its Foundation.       §19. Secondly, That in Relations, we have for the most part, if not
always, as clear a Notion of the Relation, as we have of those simple
Ideas, wherein it is founded: Agreement or Disagreement, whereon
Relation depends, being Things, whereof we have commonly as
clear Ideas, as of any other whatsoever; it being but the distin-
guishing simple Ideas, or their Degrees one from another, without
which, we could have no distinct Knowledge at all. For if I have a
clear Idea of Sweetness, Light, or Extension, I have too, of equal or
more, or less, of each of these: If I know what it is for one Man to
be born of a Woman, viz. Sempronia, I know what it is for another
Man to be born of the same Woman, Sempronia; and so have as
clear a Notion of Brothers, as of Births, and, perhaps, clearer. For if
I believed, that Sempronia digged Titus out of the Parsley-Bed, (as
they use to tell Children,) and thereby became his Mother; and that
afterwards in the same manner, she digged Cajus out of the Parsley-
Bed, I had as clear a Notion of the Relation of Brothers between
them, as if I had all the Skill of a Midwife; the Notion that the same
Woman contributed, as Mother, equally to their Births, (though I
were ignorant or mistaken in the manner of it,) being that on which
I grounded the Relation; and that they agreed in that Circumstance
of Birth, let it be what it will. The comparing them then in their
descent from the same Person, without knowing the particular
Circumstances of that descent, is enough to found my Notion of
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their having, or not having the Relation of Brothers. But though the
Ideas of particular Relations, are capable of being as clear and distinct
in the Minds of those, who will duly consider them, as those of
mixed Modes, and more determinate, than those of Substances; yet
the Names belonging to Relation, are often of as doubtful, and
incertain Signification, as those of Substances, or mixed Modes; and
much more than those of simple Ideas. Because Relative Words,
being the Marks of this Comparison, which is made only by Men’s
Thoughts, and is an Idea only in Men’s Minds, Men frequently
apply them to different Comparisons of Things, according to their
own Imaginations, which do not always correspond with those of
others using the same Names.
Locke Hum II, 28, §19, pp. 361-362