It is also to be observed, first, that seven years is the age at which the longest duration of life is to be expected, since there is then an equal chance of living 42 years and three months longer; secondly that at the age of 12 one fourth of our existence is gone, as we cannot in reason expect above 38 or 39 years more; thirdly, that we have enjoyed one half of our existence at the age of 28, as we can reckon upon only 28 years more; and lastly, that by the age of 50 three fourths of life are passed, the remaining probability being only for 16 or 17 years. But these physical truths, however mortifying, may be compensated by moral considerations. A man ought to consider as nothing the first fifteen years of his life. Every thing that happens in that long interval of time is effaced from the memory, or has at least so little connection with the views and objects Which afterwards occupy our thoughts that it gives us no concern. Neither, indeed, have we the game succession of ideas, nor, it maybe said, the same existence. In a moral sense we do not begin to live till we have begun to regulate our thoughts, to direct them towards futurity, and to assume to ourselves a kind of consistency of character conformable to that state which has some relation to what we shall afterwards become. By considering the duration of life in this, the only real point of view, we shall find, that at the age of 25 we have passed but one fourth part of our life; at the age of 38 one half; and that, at the age of 56, there is one fourth of life still remains. CHAPTER VI. OF THE SENSE OF SEEING. Having described the parts of which the human body consists, let us now proceed to examine its principal organs; the expansion of the senses, and their several functions; and at the same time, point out the errors to which, through them, we are in some measure subjected by Nature. The eyes seem to be formed very early in the human embryo. In the chicken also, of all the double organs they are the soonest produced. I have observed in the eggs of several sorts of birds, and in those of lizards, that the eyes were more large, and early in their expansion, than any other parts of a two-fold growth. Though, in viviparous animals, and particularly in man, they are, at first, by no means so large in proportion as in the oviparous, yet they obtain their due formation sooner than any other parts of the body. It is the same with the organ of hearing; the small bones of the ear are entirely formed before any of the other bones have acquired any part of their growth or solidity. Hence it is evident, that the parts of the body, which are furnished with the greatest quantity of nerves, as the ears and eyes, are those which first appear, and which are the soonest brought to perfection.