Euclid

What is proved without a diagram, is not proved at all.

And when the lines containing the angle are straight, the angle is called rectilinear.

And the point is called the centre of the circle.

A diameter of the circle is any straight line drawn through the centre and terminated in both directions by the circumference of the circle, and such a line also bisects the circle.

A semicircle is the figure contained by the diameter and the circumference cut off by it. And the centre of the semicircle is the same as that of the circle.

Rectilinear figures are those which are contained by straight lines, trilateral figures being those contained by three, quadrilateral those contained by four, and multilateral those contained by more than four straight lines.

To describe a circle with any centre and radius.

That all right angles are equal to one another.

That, if a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles.

The knowledge of which geometry is the study is eternal.

Geometry is the gate of science, and the entrance of the mathematics.

The art of measuring is the most useful and the most necessary of all arts.

Mathematics is the queen of the sciences and arithmetic the queen of mathematics.

Give me a firm spot on which to stand, and I will move the earth.

The object of geometry is to throw light on the invisible.

The study of mathematics, like the Nile, begins in minuteness but ends in magnificence.

Truth is eternal, and so is geometry.

The beautiful is in the eye of the beholder, but the truth is in the proof.

Let it be granted that a finite straight line may be produced continuously in a straight line.

Let it be granted that a circle may be described with any centre and distance.