Elsewhere I have distinguished philosophy from mathematics, from history, and from all the empirical sciences, by declaring philosophy to be a noninvestigative discipline that employs the evidence of common experience. Unlike history and the empirical sciences that employ the evidence of special experience discovered by their specifically investigative methods, philosophy is strictly armchair thinking.
But it may be said also that mathematics is armchair thinking. Like the philosopher, the mathematician does not have to leave the desk for a moment to carry on any investigations in order to engage in mathematical thought. But unlike the philosopher, the mathematician is not concerned with objects that have any existence in a reality independent of our minds.
This fourfold array of the disciplines -- philosophy, mathematics, history, and the empirical sciences -- distinguishes them from one another by their characteristically different methods. Questions that can be answered by one method cannot be answered by the other methods. Some questions may not be purely scientific or purely philosophical. But if a question is purely philosophical, then this is a question to which the methods of the other three disciplines cannot contribute.
This approach enables us to explain how in modern times, when philosophy is dismissed by logical positivist as giving no truths about an independent reality, the invidious dismissal of philosophy as mere opinion can be countered by showing the way in which there is progress in philosophy as well as in science, agreement in philosophy as well as in science, a practical utility of philosophy as well as of science.
In any case, the progress, the agreements, and the use of philosophy will be characteristically different from that which occurs in the empirical sciences.
We are also enabled to deal with the history of philosophy, in ancient, medieval, and modern times, by reference not to the various doctrines espoused in these different epochs, not only by reference to the slow development of the method that is truly philosophical. This I call a purely procedural, not substantive, history of philosophy; and from this procedural point of view, philosophy has achieved its maturity only in recent years.
The four dimensions of philosophy are metaphysical, moral, objective and categorical.