And I had little difficulty in determining the objects with which it was necessary to commence, for I was already persuaded that it must be with the simplest and easiest to know, and, considering that of all those who have hitherto sought truth in the sciences, the mathematicians alone have been able to find any demonstrations, that is, any certain and evident reasons, I did not doubt but that such must have been the rule of their investigations.
I should not, however, on this account have ventured at once on the examination of all the difficulties of the sciences which presented themselves to me, for this would have been contrary to the order prescribed in the method, but observing that the knowledge of such is dependent on principles borrowed from philosophy, in which I found nothing certain, I thought it necessary first of all to endeavor to establish its principles.
In the vertebrata, we see a series of internal vertebrae bearing certain processes and appendages; in the articulata, we see the body divided into a series of segments, bearing external appendages; and in flowering plants, we see a series of successive spiral whorls of leaves.
-- It has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike.
Even as he did so he thought he perceived a certain haziness of eye and speech in his trustee; but he was too hopeful to be stayed, silenced the voice of warning in his bosom, and with one and the same gesture committed the money to the clerk, and himself into the hands of destiny.
Meanwhile, John had gone upon his holidays without a word, which was irregular; and there had disappeared with him a certain sum of money, which was out of all bounds of palliation.
Unless you adopted the opinion of certain observers of the human heart, and thought that the chevalier had the voice of his nose, his organ of speech would have amazed you by its full and redundant sound.
The chevalier had repudiated the ridiculous costume still preserved by certain monarchical old men; he had frankly modernized himself.
Razumov at a certain address in town which seemed to be that of an oculist.
At any rate, he went; but, what's more, he went with a certain eagerness, which may appear incredible till it is remembered that Councillor Mikulin was the only person on earth with whom Razumov could talk, taking the Haldin adventure for granted.
Wordsworth's own character, as we have already observed, was dominated by a certain contentment, a sort of naturally religious placidity, not often found in union with a poetic sensibility so  active as his; and this gentle sense of well-being was favourable to the quiet, habitual observation of the inanimate, or imperfectly animate, world.
An emanation, a particular spirit, belonged not to the moving leaves or water only, but to the distant peak arising suddenly, by some change of perspective, above the nearer horizon of the hills, to the passing space of light across the plain, to the lichened Druidic stone even, for a certain weird fellowship in it with the moods of men.
I never heard him speak of those whose work had a certain
analogy with his own -- of Cezanne, for instance, or of Van Gogh; and I doubt very much if he had ever seen their pictures.
Those words only denote a certain
stage of understanding of phenomena.
He decides, let us suppose, to go to a certain
place, whereupon he proceeds to the station, takes his ticket and enters the train.