The scientists of the Renaissance brought about the most fundamental alterations in the world of thought, and they accomplished this feat by devising a new method for discovering knowledge. Unlike the medieval thinkers, who proceeded for the most part by reading traditional texts, the early modern scientists laid greatest stress upon observation and the formation of temporary hypothesis. The method of observation implied two things: namely, those traditional explanations of the behavior of nature should be empirically demonstrated, the new assumption being that such explanations could very well be wrong, and that new information might be available to scientists so they could penetrate beyond the superficial appearances of things. People now began to look at the heavenly bodies with a new attitude, hoping not solely to find the confirmation of Biblical statements about the firmament but, further, to discover the principles and laws that describe the movements of bodies. Observation was directed not only upon the stars but also in the opposite direction, toward the minutest constituents of physical substance.

To enhance the exactness of their observations, they invented various scientific instruments. Lippershey, a Dutchman, invented the telescope in 1608, although Galileo was the first to make dramatic use of it. In 1590 the first compound microscope was created. The principle of the barometer was discovered by Galileo's pupil Torricelli. The air pump, which was so important in creating a vacuum for the experiment that, proved that all bodies regardless of their weight or size fall at the same rate when there is no air resistance, was invented by Otto von Guericke(1602 -1686). With the use of instruments and imaginative hypotheses, fresh knowledge began to unfold. Galileo discovered the moons around Jupiter, and Anton Leeuwenhoek (1632 -1723) discovered spermatozoa, protozoa, and bacteria.

Whereas Nicolas Copernicus (1473 -1543) formed a new hypothesis of the revolution of, the earth around the sun, Harvey (1578 - 1657) discovered the circulation of the blood. William Gilbert (1540 -1603) wrote a major work on the magnet, and Robert Boyle (1627 -1691), the father of chemistry, formulated his famous law concerning the relation of temperature, volume, and pressure of gases. Added to these inventions and discoveries was the decisive advance made in mathematics, especially by Sir Isaac Newton and Leibniz, who independently invented differential and integral calculus. The method of observation and mathematical calculation now became the hallmarks of modern science.

The new scientific mode of thought in time influenced philosophic thought in two important ways. First, the assumption that the basic processes of nature are observable and capable of mathematical calculation and description had the effect of engendering another assumption, namely, that everything consists of bodies in motion, that everything conforms to a mechanical model. The heavens above and the smallest particles below all exhibit the same laws of motion. Even human thought was soon explained in mechanical terms, not to mention the realm of human behavior, which the earlier moralists described as the product of free will.