How did the object of study in psychology change over the history of the field since the 19th century?

Newspaper started by john campbell:

Which statement is a good description of a model? a) models are a description of nature based only on one or two variables. b) models are testable ideas but they are not very to most people. c) models are a description of nature that predicts more accurately than real events. d) models are an approximiation of a real system that omits all but most essential variables in a system.

What was the greatest contribution of the “scientific method”?