Descartes confined science to the study of the physical realm of reality, the realm of matter and energy, of things that can be measured, of things that operate in purely mechanistic terms.
After Descartes, scientists had such success in explaining the world in purely physical terms, that they eventually dropped the the mental realm from their model of reality. Essentially, many scientists decided that as the mental realm was unmeasurable, it didn't exist, or at least, that it wasn't important. They stated that there was no 'ghost running the machine', there was simply...the machine.
Much later, in psychology, a strongly mechanistic perspective was adopted by behaviorism (described later), and is also evident in the use of medical interventions (e.g. drugs and electro-shock therapy) to treat mental disorders. In a mechanistic perspective the mind is viewed as a mere byproduct of the mechanical operations of the brain, with no control over our behavior. The mind is like a sports announcer in a booth at a football game, noticing and commenting, but not actually affecting what is going on in the field of action.