Humanism is a general term for many different lines of thought that focus on humanity and issues that are common to human beings. Many forms of humanism take as their starting point the doctrine of Protagoras that "man is the measure of all things". This statement is commonly understood to mean that people, not objective or absolutist factors, determine a thing’s worth. Democratic societies in which the citizens themselves choose the laws and values that govern them are founded on this humanist principle; however, the principle is typically too radical for many thinkers, humanists among them, and the different strains of humanism reflect the different "yes, but" qualifiers that people place on the Protagoran formula.

Renaissance humanism was a cultural movement in Europe beginning in central Italy in the 14th century, that revived and refined the study of the language (in particular the Greek language), science, philosophy, art and poetry of classical antiquity. Their emphasis on art and the senses marked a great change from the medieval values of humility, introspection, and passivity. Beauty was held to represent a deeper inner virtue and value. The crisis of Renaissance humanism came with the trial of Galileo, for it forced the choice between basing the authority of one's beliefs on one's observations or upon religious teaching. The trial made the contradictions between humanism and religion visible to all and made humanism a dangerous doctrine.

Renaissance humanism was an aristocratic movement, not at all a democratic one, and it has always had opponents who saw it as a corrupting, luxurious doctrine. Nevertheless, the appeal of humanist accomplishment has always been strong, and its patronage of the arts assured that it would find a place in the artisan class. With the spread of printing and the appearance of the intellectual writer, a middle-class humanist also appeared, and the Enlightenment can be viewed as the spread of humanist values beyond the aristocracy. The Enlightenment tended to present science and reason, more than art, as the defining trait of human dignity. Enlightenment humanists, perhaps more than any other group, took their Protagoras straight and did not offer many qualifiers to his principle.

Modern humanism has two branches. One stems from the Renaissance-Enlightenment tradition. It contains many artists, mainline Protestants, and scholars in the liberal arts. Their view tends to concentrate on the dignity and nobility of human achievement and possibility. The other branch reflects the rise of globalism, technology and the collapse of religious authority. It is characterized by an attitude centered on human interests or values, stressing an individual's dignity and worth and capacity for self-realization through reason and logic. It is much more comfortable with Puritanism's distaste for luxury than is traditional humanism, and its emphasis on the scientific limits of human capacity are also more in keeping with traditional, anti-humanist religious doctrines, although these secular humanists reject religion and supernaturalism of any form. They see themselves as providing an answer to the need for a common philosophy that transcends the cultural boundaries of local moral codes and religions.

Many people call themselves humanists of one form or another. Some religious people consider themselves humanists because their religious beliefs are moral, and therefore humane. Humanism is also used sometimes, but probably incorrectly, to describe humanities scholars (particularly classicists) or as a synonym for "humanitarianism".

Humanism as a current in education began to dominate school systems in the 19th Century. It held that the studies that develop our intellect are those that make us most truly human. Assimilationist, stern, and rigorous, the aim was to bring the affective and psychomotor natures under the control of the intellect. The practical basis for this was faculty psychology, or the belief in distinct intellectual faculties such as the analytical, the mathematical, the linguistic, etc. Strengthening one faculty was believed to help other faculties as well (transfer of training). A key player in the late 19th century educational humanism was U.S. Commissioner of Education W.T. Harris, whose "Five Windows of the Soul" (math, geography, history, grammar, and literature/art) were believed especially appropriate for development of the faculties. Educational humanists believe that the best studies for the best kids are the best studies for all kids. While humanism as an educational current was largely discredited by the innovations of the early 20th century, it still holds out in some elite preparatory schools and some high school disciplines (especially, of course, literature).