At its most general, Idealism asserts that the physical world is less important than the mind or the spirit which shapes and animates it. Idealists choose the soul, the mind or the psyche over the body, the material and the historical. When ideals (of appearance, or proportion for example) regulate the way an artist represents the world his work can be described as Idealist.
Plato's Theory of the Forms was the the most important Classical influence on Renaissance Idealism. The Ideas, or Forms, contain all that is necessary and universal and are therefore perfect and unchanging while the material world is simply a deceptive procession of changing appearances which have no more reality than shadows.
The leading artists of the High Renaissance--Leonardo, Raphael and Michelangelo --are all associated with slightly different forms of Idealism. Michelangelo's was most identified with the Platonic Forms because of his reliance on ‘disegno’(design). The term is used to describe art which is shaped by the artist's imagination rather than by his imitation, or copying, of a natural model. Michelangelo's art also idealised the body by giving it monumental proportions. His figures are usually astonishingly muscular.
Michaelangelo - Leda and the Swan (1530)
Raphael's figures are equally idealised but they are characterised by sweetness of expression, serenity, elegance, clarity of line and beauty of colour rather than physical grandeur.
Raphael - Saint Catherine of Alexandria (1508)
Leonardo's Idealism was different again. It was characterised by an emphasis on finding the Divine in the perfectly human. Beauty, subtle facial expression, the unity of the figure with its setting and the elimination of unnecessary detail to suggest emotion, were all aspects of his Idealism.
Leonardo di Vinci - Benois Madonna (1478)
Renaissance artists strove to paint the Ideal, not because they shied away from looking at the world, but because they wanted to capture the absolute and universal truth hidden within it.