something which sees the brown colour is quite momentary, and not the
same as the something which has some different experience the next
moment.

Thus it is our particular thoughts and feelings that have primitive
certainty. And this applies to dreams and hallucinations as well as
to normal perceptions: when we dream or see a ghost, we certainly do
have the sensations we think we have, but for various reasons it is
held that no physical object corresponds to these sensations. Thus
the certainty of our knowledge of our own experiences does not have to
be limited in any way to allow for exceptional cases. Here,
therefore, we have, for what it is worth, a solid basis from which to
begin our pursuit of knowledge.

The problem we have to consider is this: Granted that we are certain
of our own sense-data, have we any reason for regarding them as signs
of the existence of something else, which we can call the physical
object? When we have enumerated all the sense-data which we should
naturally regard as connected with the table, have we said all there
is to say about the table, or is there still something else--something
not a sense-datum, something which persists when we go out of the
room? Common sense unhesitatingly answers that there is. What can be
bought and sold and pushed about and have a cloth laid on it, and so
on, cannot be a _mere_ collection of sense-data. If the cloth
completely hides the table, we shall derive no sense-data from the
table, and therefore, if the table were merely sense-data, it would
have ceased to exist, and the cloth would be suspended in empty air,
resting, by a miracle, in the place where the table formerly was.
This seems plainly absurd; but whoever wishes to become a philosopher
must learn not to be frightened by absurdities.