Unlike earlier great thinkers, Rene Descartes was the first to view psychology as a form of science in its own way. Dissatisfied with what he was taught in school, he took a particular interest in studying psychology and its similarity to scientific principles. However, he approaches psychological science differently from other science disciplines that require physical or mechanical parameters. His invention of the Cartesian plane and coordinates was the first breakthrough at associating psychology with science (Stanford). Just like the possibility to describe the relationship that exists between dependent and independent variables in science, psychology uses the coordinate system to define the relation of different phenomena. Descartes suggested that it was possible to perform significant tests, calculate correlations and other quantitative analysis of psychological parameters differently from disciplines like physics.
Descartes incorporated the use of mathematic-like certainty principles in the determination of psychological solutions. He came up with four scientific rules that would define psychology (Stanford). The first rule was to proceed without doubt in solving psychological problems, to avoid prejudgment of situations and to take no parameters for granted. This could be associated with scientific procedures that normally consider all parameters that could have confounding or supporting effects on a problem being solved. Unlike other sciences like physics, these parameters do not need to be physical indicators which could be measured.
The second rule involved dividing the subject of argument into simpler parts that can be solved independently. This is based on the scientific principle of fractionating complex parameters with the aim of reducing them into fundamental variables that can be easily solved. This allows psychological variables to be assessed independently and easily to establish relationships that exist between these simpler parts.