Final answer:
Psychology became a separate scientific discipline when Wilhelm Wundt opened his experimental psychology laboratory in Leipzig in 1879.
Step-by-step explanation:
Psychology transitioned from philosophy to science primarily with the event when Wilhelm Wundt opened his laboratory in Leipzig in 1879. This laboratory was the first of its kind for psychological research and is where Wundt and his students conducted various experiments, particularly on reaction times to different stimuli. The establishment of this laboratory marked the beginning of experimental psychology as a separate discipline, distinct from philosophy, contributing significantly to the development of psychology as a science.