[1665-08-16] ...Hence to the Exchange, which I have not been a great while. But Lord, how sad a sight it is to see the streets empty of people, and very few upon the Change - jealous of every door that one sees shut up, lest it should be the plague - and about us, two shops in three, if not more, generally shut up. ... It was dark before I could get home; and so land at church-yard stairs, where to my great trouble I met a dead Corps, of the plague, in the narrow ally, just bringing down a little pair of stairs - but I thank God I was not much disturbed at it. However, I shall beware of being late abroad again.

A learned man is an idler who kills time with study. Beware of his false knowledge: it is more dangerous than ignorance.

Amoeba has her picture in the book,
Proud Protozoon!—Yet beware of pride,
All she can do is fatten and divide;
She cannot even read, or sew, or cook…
The Worm can crawl
But has no eyes to look.
The Jelly-fish can swim
But lacks a bride.

Beware of finding what you're looking for.
A favorite aphorism he often used.

Beware of old Linnaeus,
The Man of the Linden-tree,
So beautiful, bright and early
He brushed away the dews
He found the wicked wild-flowers
All courting there in twos.

Beware of the problem of testing too many hypotheses; the more you torture the data, the more likely they are to confess, but confessions obtained under duress may not be admissible in the court of scientific opinion.

Beware of the young Doctor, & the old Barber.

Beware when the great God lets loose a thinker on this planet. Then all things are at risk. ... There is not a piece of science, but its flank may be turned to-morrow.

I was sitting writing at my textbook but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gambolling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by the repeated visions of the kind, could now distinguish larger structures of manifold confirmation: long rows, sometimes more closely fitted together all twining and twisting in snake like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the rest of the hypothesis. Let us learn to dream, gentlemen, then perhaps we shall find the truth... But let us beware of publishing our dreams till they have been tested by waking understanding.

I would by all means have men beware, lest Æsop’s pretty fable of the fly that sate [sic] on the pole of a chariot at the Olympic races and said, “What a dust do I raise,” be verified in them. For so it is that some small observation, and that disturbed sometimes by the instrument, sometimes by the eye, sometimes by the calculation, and which may be owing to some real change in the heaven, raises new heavens and new spheres and circles.

In medicine … beware of ambiguity.

The conservative has but little to fear from the man whose reason is the servant of his passions, but let him beware of him in whom reason has become the greatest and most terrible of the passions.

The Dark Ages may return—the Stone Age may return on the gleaming wings of Science; and what might now shower immeasureable material blessings upon mankind may even bring about its total destruction. Beware! I say. Time may be short.
Referring to the discovery of atomic energy.

The good Christian should beware of mathematicians, and all those who make empty prophecies. The danger already exists that the mathematicians have made a covenant with the devil to darken the spirit and to confine man in the bonds of Hell.

The words figure and fictitious both derive from the same Latin root, fingere. Beware!

There is no supernatural, there is only nature. Nature alone exists and contains all. All is. There is the part of nature that we perceive, and the part of nature that we do not perceive. … If you abandon these facts, beware; charlatans will light upon them, also the imbecile. There is no mean: science, or ignorance. If science does not want these facts, ignorance will take them up. You have refused to enlarge human intelligence, you augment human stupidity. When Laplace withdraws Cagliostro appears.