[15] (1) We must seek the assistance of Moral Philosophy [d] and the Theory of Education; further, as health is no insignificant means for attaining our end, we must also include the whole science of Medicine, and, as many difficult things are by contrivance rendered easy, and we can in this way gain much time and convenience, the science of Mechanics must in no way be despised.
[16] (1) But before all things, a means must be devised for improving the understanding and purifying it, as far as may be at the outset, so that it may apprehend things without error, and in the best possible way. (2) Thus it is apparent to everyone that I wish to direct all science to one end [e] and aim, so that we may attain to the supreme human perfection which we have named; and, therefore, whatsoever in the sciences does not serve to promote our object will have to be rejected as useless. (3) To sum up the matter in a word, all our actions and thoughts must be directed to this one end.
[17] (1) Yet, as it is necessary that while we are endeavoring to attain our purpose, and bring the understanding into the right path we should carry on our life, we are compelled first of all to lay down certain rules of life as provisionally good, to wit the following:--
I. (2) To speak in a manner intelligible to the multitude, and to comply with every general custom that does not hinder the attainment of our purpose. (3) For we can gain from the multitude no small advantages, provided that we strive to accommodate ourselves to its understanding as far as possible: moreover, we shall in this way gain a friendly audience for the reception of the truth.
II. (17:4) To indulge ourselves with pleasures only in so far as they are necessary for preserving health.
III. (5) Lastly, to endeavor to obtain only sufficient money or other commodities to enable us to preserve our life and health, and to follow such general customs as are consistent with our purpose.
[18] (1) Having laid down these preliminary rules, I will betake myself to the first and most important task, namely, the amendment of the understanding, and the rendering it capable of understanding things in the manner necessary for attaining our end. (2) In order to bring this about, the natural order demands that I should here recapitulate all the modes of perception, which I have hitherto employed for affirming or denying anything with certainty, so that I may choose the best, and at the same time begin to know my own powers and the nature which I wish to perfect.
[19] (1) Reflection shows that all modes of perception or knowledge may be reduced to four:--
I. (2) Perception arising from hearsay or from some sign which everyone may name as he please.
II. (3) Perception arising from mere experience--that is, form experience not yet classified by the intellect, and only so called because the given event has happened to take place, and we have no contradictory fact to set against it, so that it therefore remains unassailed in our minds.