The Complete Aristotle (eng.)

treatise
about the soul, for it belongs to an investigation distinct from
that which lies before us.
    As there are in the mind thoughts which do not involve truth or
falsity, and also those which must be either true or false, so it
is in speech. For truth and falsity imply combination and
separation. Nouns and verbs, provided nothing is added, are like
thoughts without combination or separation; ‘man’ and ‘white’, as
isolated terms, are not yet either true or false. In proof of this,
consider the word ‘goat-stag.’ It has significance, but there is no
truth or falsity about it, unless ‘is’ or ‘is not’ is added, either
in the present or in some other tense.
2
    By a noun we mean a sound significant by convention, which has
no reference to time, and of which no part is significant apart
from the rest. In the noun ‘Fairsteed,’ the part ‘steed’ has no
significance in and by itself, as in the phrase ‘fair steed.’ Yet
there is a difference between simple and composite nouns; for in
the former the part is in no way significant, in the latter it
contributes to the meaning of the whole, although it has not an
independent meaning. Thus in the word ‘pirate-boat’ the word ‘boat’
has no meaning except as part of the whole word.
    The limitation ‘by convention’ was introduced because nothing is
by nature a noun or name-it is only so when it becomes a symbol;
inarticulate sounds, such as those which brutes produce, are
significant, yet none of these constitutes a noun.
    The expression ‘not-man’ is not a noun. There is indeed no
recognized term by which we may denote such an expression, for it
is not a sentence or a denial. Let it then be called an indefinite
noun.
    The expressions ‘of Philo’, ‘to Philo’, and so on, constitute
not nouns, but cases of a noun. The definition of these cases of a
noun is in other respects the same as that of the noun proper, but,
when coupled with ‘is’, ‘was’, or will be’, they do not, as they
are, form a proposition either true or false, and this the noun
proper always does, under these conditions. Take the words ‘of
Philo is’ or ‘of or ‘of Philo is not’; these words do not, as they
stand, form either a true or a false proposition.
3
    A verb is that which, in addition to its proper meaning, carries
with it the notion of time. No part of it has any independent
meaning, and it is a sign of something said of something else.
    I will explain what I mean by saying that it carries with it the
notion of time. ‘Health’ is a noun, but ‘is healthy’ is a verb; for
besides its proper meaning it indicates the present existence of
the state in question.
    Moreover, a verb is always a sign of something said of something
else, i.e. of something either predicable of or present in some
other thing.
    Such expressions as ‘is not-healthy’, ‘is not, ill’, I do not
describe as verbs; for though they carry the additional note of
time, and always form a predicate, there is no specified name for
this variety; but let them be called indefinite verbs, since they
apply equally well to that which exists and to that which does
not.
    Similarly ‘he was healthy’, ‘he will be healthy’, are not verbs,
but tenses of a verb; the difference lies in the fact that the verb
indicates present time, while the tenses of the verb indicate those
times which lie outside the present.
    Verbs in and by themselves are substantival and have
significance, for he who uses such expressions arrests the hearer’s
mind, and fixes his attention; but they do not, as they stand,
express any judgement, either positive or negative. For neither are
‘to be’ and ‘not to be’ the participle ‘being’ significant of any
fact, unless something is added; for they do not themselves
indicate anything, but imply a copulation, of which we cannot form
a conception apart from the things coupled.
4
    A sentence is a significant portion of speech, some parts of
which have an independent meaning, that is to say, as an utterance,
though not as the expression of any positive judgement. Let me
explain. The word ‘human’ has meaning, but does not constitute a
proposition, either positive or negative. It is only when other
words are added that the whole will form an affirmation or denial.
But if we separate one syllable of the word ‘human’ from the other,
it has no meaning; similarly in the word ‘mouse’, the part ‘ouse’
has no meaning in itself, but is merely a sound.