No one would deny that the painter has nothing to do with things that are not visible. The painter is concerned solely with representing what can be seen. - Leon Battista Alberti, 1435

In the Middle Ages, the period before the Renaissance, most art in Europe featured heavenly figures devoted to the worship of Christ. Because the people in Medieval paintings were citizens of heaven and the artists painting these pictures had never actually seen heaven, the background was left to the imagination and the teachings of the church. Gold backgrounds were very common, as the air in heaven surely must be precious. When people became more interested in the world around them and the ideas of other people rather than heaven and the teachings of Christ and the saints, landscapes and buildings began to show up in paintings. Everyone could see landscapes and buildings everyday so one of the essential artistic problems of the Renaissance became how to paint landscapes and buildings in pictures so that they looked the same as in real life.

Figure 4, Filippo Brunelleschi, Linear Perspective. Drawing by Silvia Minguzzi, 2013.

In the Renaissance, painters needed to be able to translate the three-dimensional world around them onto the two-dimensional surface of a painting, called the picture plane. The solution was linear perspective (Figure 4); the idea that converging lines meet at a single vanishing point and all shapes get smaller in all directions with increasing distance from the eye. The discovery of perspective is attributed to the architect Filippo Brunelleschi (1377-1446), who suggested a system that explained how objects shrink in size according to their position and distance from the eye. However, the nature of Brunelleschi's system and date of its discovery remain unclear. In 1435, Leon Battista Alberti (1404-1472), provided the first theory of what we now call linear perspective in his book, On Painting 2 .

The impact of this new system of measurement in paintings was enormous and most artists painting in Europe after 1435 were aware of the principles Alberti outlined in his book.
First, an artist created a "floor" (a ground or stage on which figures and objects would be placed) in a painting and drew a receding grid to act as a guide to the relative scale of all other elements within the picture (Figure 5).

Figure 5, Leon Battista Alberti's perspective. Drawing by Silvia Minguzzi, 2013

Alberti suggests relating the size of the floor squares to a viewer's height. This suggestion is important because it reveals an underlying principal of the Renaissance. The act of painting would no longer be to glorify God, as it hadbeen in Medieval Europe. Painting in the Renaissance related instead, to those people looking at the painting.