When the plain man sees “a table” in the presence of a philosopher, the plain man can be driven, by the arguments we have repeatedly brought forward, to admit that he cannot have complete certainty as to anything outside himself. But if he does not lose his head or his temper, he will remain certain that there is a coloured pattern, which may be in him, but indubitably exists. No argument from logic or physics even tends to show that he is mistaken in this; therefore there is no reason why he should surrender his conviction. The argument about knowledge in Chapter VIII showed that, accepting the usual views of physicists as to causal laws, our knowledge becomes more certain as the causal chain from object to reaction is shortened, and can only be quite certain when the two are in the same place in space-time, or at least contiguous. Thus we should expect that the highest grade of certainty would belong to knowledge as to what happens in our own heads. And this is exactly what we have when we are aware of our own “mental” events, such as the existence of a coloured pattern when we thought we were seeing a table. We might, therefore, if we were anxious to preserve the word “mental”, define a “mental” event as one that can be known with the highest grade of certainty, because, in physical space-time, the event and the knowing of it are contiguous. Thus “mental” events will be certain of the events that occur in heads that have brains. They will not be _all_ events that occur in brains, but only such as cause a reaction of the kind that can be called “knowledge”. There are, however, still a number of difficult questions, to which, as yet, a definitive answer cannot be given. When we “know” a thought of our own, what happens? And do we know the thought in a more intimate way than we know anything else? Knowledge of external events, as we have seen, consists of a certain sensitivity to their presence, but not in having in or before our minds anything similar to them, except in certain abstract structural respects. Is knowledge of our own minds equally abstract and indirect? Or is it something more analogous to what we ordinarily imagine knowledge to be? Take first the question: What happens when we “know” a thought of our own? Taking the definition of “knowing” that we adopted in Chapter VIII, we shall say: We “know” a thought of our own when an event in our brain causes a characteristic reaction which is present when the event occurs and not otherwise. In this sense, whenever we say, “I see a table”, we are knowing a thought, since an event in our brain is the only invariable antecedent of such a statement (assuming it to be made truthfully). We may _think_ we are knowing a table, but this is an error.