ion applies, though the object itself is
not directly known to us. In such a case, we say that our knowledge of
the object is knowledge by description.

All our knowledge, both knowledge of things and knowledge of truths,
rests upon acquaintance as its foundation. It is therefore important to
consider what kinds of things there are with which we have acquaintance.

Sense-data, as we have already seen, are among the things with which
we are acquainted; in fact, they supply the most obvious and striking
example of knowledge by acquaintance. But if they were the sole example,
our knowledge would be very much more restricted than it is. We should
only know what is now present to our senses: we could not know anything
about the past--not even that there was a past--nor could we know any
truths about our sense-data, for all knowledge of truths, as we shall
show, demands acquaintance with things which are of an essentially
different character from sense-data, the things which are sometimes
called 'abstract ideas', but which we shall call 'universals'. We have
therefore to consider acquaintance with other things besides sense-data
if we are to obtain any tolerably adequate analysis of our knowledge.

The first extension beyond sense-data to be considered is acquaintance
by _memory_. It is obvious that we often remember what we have seen or
heard or had otherwise present to our senses, and that in such cases we
are still immediately aware of what we remember, in spite of the fact
that it appears as past and not as present. This immediate knowledge by
memory is the source of all our knowledge concerning the past: without
it, there could be no knowledge of the past by inference, since we
should never know that there was anything past to be inferred.

The next extension to be considered is acquaintance by _introspection_.
We are not only aware of things, but we are often aware of being aware
of them. When I see the sun, I am often aware of my seeing the sun; thus
'my seeing the sun' is an object with which I have acquaintance. When
I desire food, I may be aware of my desire for food; thus 'my desiring
food' is an object with which I am acquainted. Similarly we may be
aware of our feeling pleasure or pain, and generally of the events which
happen in our minds. This kind of acquaintance, which may be called
self-consciousness, is the source of all our knowledge of mental things.
It is obvious that it is only what goes on in our own