There are many senses in which a thing may be said to "be", but
all that "is" is related to one central point, one definite kind of
thing, and is not said to "be" by a mere ambiguity. Everything which
is healthy is related to health, one thing in the sense that it
preserves health, another in the sense that it produces it, another in
the sense that it is a symptom of health, another because it is
capable of it. And that which is medical is relative to the medical
art, one thing being called medical because it possesses it, another
because it is naturally adapted to it, another because it is a
function of the medical art. And we shall find other words used
similarly to these. So, too, there are many senses in which a thing is
said to be, but all refer to one starting-point; some things are
said to be because they are substances, others because they are
affections of substance, others because they are a process towards
substance, or destructions or privations or qualities of substance, or
productive or generative of substance, or of things which are relative
to substance, or negations of one of these thing of substance
itself. It is for this reason that we say even of non-being that it is
nonbeing. As, then, there is one science which deals with all
healthy things, the same applies in the other cases also. For not only
in the case of things which have one common notion does the
investigation belong to one science, but also in the case of things
which are related to one common nature; for even these in a sense have
one common notion. It is clear then that it is the work of one science
also to study the things that are, qua being.-But everywhere science
deals chiefly with that which is primary, and on which the other
things depend, and in virtue of which they get their names. If,
then, this is substance, it will be of substances that the philosopher
must grasp the principles and the causes.

    Now for each one class of things, as there is one perception, so
there is one science, as for instance grammar, being one science,
investigates all articulate sounds. Hence to investigate all the
species of being qua being is the work of a science which is
generically one, and to investigate the several species is the work of
the specific parts of the science.

    If, now, being and unity are the same and are one thing in the
sense that they are implied in one another as principle and cause are,
not in the sense that they are explained by the same definition
(though it makes no difference even if we suppose them to be like
that-in fact this would even strengthen our case); for "one man" and
"man" are the same thing, and so are "existent man" and "man", and the
doubling of the words in "one man and one existent man" does not
express anything different (it is clear that the two things are not
separated either in coming to be or in ceasing to be); and similarly
"one existent man" adds nothing to "existent man", and that it is
obvious that the addition in these cases means the same thing, and
unity is nothing apart from being; and if, further, the substance of
each thing is one in no merely accidental way, and similarly is from
its very nature something that is:-all this being so, there must be
exactly as many species of being as of unity. And to investigate the
essence of these is the work of a science which is generically one-I
mean, for instance, the discussion of the same and the similar and the
other concepts of this sort; and nearly all contraries may be referred
to this origin; let us take them as having been investigated in the
"Selection of Contraries".

    And there are as many parts of philosophy as there are kinds of
substance, so that there must necessarily be among them a first
philosophy and one which follows this. For being falls immediately
into genera; for which reason the sciences too will correspond to
these genera. For the philosopher is like the mathematician, as that
word is used; for mathematics also has parts, and there is a first and
a second science and other successive ones within the sphere of
mathematics.

    Now since it is the work of one science to investigate
opposites, and plurality is opposed to unity-and it belongs to one
science to investigate the negation and the privation because in
both cases we are really investigating the one thing of which the
negation or the privation is a negation or privation (for we either
say simply that that thing is not present, or that it is not present
in some particular class; in the latter case difference is present
over and above what is implied in negation; for negation means just
the absence of the thing in question, while in privation there is also
employed an underlying nature of which the privation is
asserted):-in view of all these facts, the contraries of the
concepts we named above, the other and the dissimilar and the unequal,
and everything else which is derived either from these or from
plurality and unity, must fall within the province of the science
above named. And contrariety is one of these concepts; for contrariety
is a kind of difference, and difference is a kind of otherness.
Therefore, since there are many senses in which a thing is said to
be one, these terms also will have many senses, but yet it belongs
to one science to know them all; for a term belongs to different
sciences not if it has different senses, but if it has not one meaning
and its definitions cannot be referred to one central meaning. And
since all things are referred to that which is primary, as for
instance all things which are called one are referred to the primary
one, we must say that this holds good also of the same and the other
and of contraries in general; so that after distinguishing the various
senses of each, we must then explain by reference to what is primary
in the case of each of the predicates in question, saying how they are
related to it; for some will be called what they are called because
they possess it, others because they produce it, and others in other
such ways.

    It is evident, then, that it belongs to one science to be able
to give an account of these concepts as well as of substance (this was
one of the questions in our book of problems), and that it is the
function of the philosopher to be able to investigate all things.
For if it is not the function of the philosopher, who is it who will
inquire whether Socrates and Socrates seated are the same thing, or
whether one thing has one contrary, or what contrariety is, or how
many meanings it has? And similarly with all other such questions.
Since, then, these are essential modifications of unity qua unity
and of being qua being, not qua numbers or lines or fire, it is
clear that it belongs to this science to investigate both the
essence of these concepts and their properties. And those who study
these properties err not by leaving the sphere of philosophy, but by
forgetting that substance, of which they have no correct idea, is
prior to these other things. For number qua number has peculiar
attributes, such as oddness and evenness, commensurability and
equality, excess and defect, and these belong to numbers either in
themselves or in relation to one another. And similarly the solid
and the motionless and that which is in motion and the weightless
and that which has weight have other peculiar properties. So too there
are certain properties peculiar to being as such, and it is about
these that the philosopher has to investigate the truth.-An indication
of this may be mentioned: dialecticians and sophists assume the same
guise as the philosopher, for sophistic is Wisdom which exists only in
semblance, and dialecticians embrace all things in their dialectic,
and being is common to all things; but evidently their dialectic
embraces these subjects because these are proper to philosophy.-For
sophistic and dialectic turn on the same class of things as
philosophy, but this differs from dialectic in the nature of the
faculty required and from sophistic in respect of the purpose of the
philosophic life. Dialectic is merely critical where philosophy claims
to know, and sophistic is what appears to be philosophy but is not.

    Again, in the list of contraries one of the two columns is
privative, and all contraries are reducible to being and non-being,
and to unity and plurality, as for instance rest belongs to unity
and movement to plurality. And nearly all thinkers agree that being
and substance are composed of contraries; at least all name contraries
as their first principles-some name odd and even, some hot and cold,
some limit and the unlimited, some love and strife. And all the others
as well are evidently reducible to unity and plurality (this reduction
we must take for granted), and the principles stated by other thinkers
fall entirely under these as their genera. It is obvious then from
these considerations too that it belongs to one science to examine
being qua being. For all things are either contraries or composed of
contraries, and unity and plurality are the starting-points of all
contraries. And these belong to one science, whether they have or have
not one single meaning. Probably the truth is that they have not;
yet even if "one" has several meanings, the other meanings will be
related to the primary meaning (and similarly in the case of the
contraries), even if being or unity is not a universal and the same in
every instance or is not separable from the particular instances (as
in fact it probably is not; the unity is in some cases that of
common reference, in some cases that of serial succession). And for
this reason it does not belong to the geometer to inquire what is
contrariety or completeness or unity or being or the same or the
other, but only to presuppose these concepts and reason from this
starting-point.--Obviously then it is the work of one science to
examine being qua being, and the attributes which belong to it qua
being, and the same science will examine not only substances but
also their attributes, both those above named and the concepts "prior"
and "posterior", "genus" and "species", "whole" and "part", and the
others of this sort.