One of the big questions in psychology is when and how psychology disentangled from philosophy. Usually it is referred to the laboratory Wundt established in Leipzig in 1879 as the birth for psychology as an independent science. However this separation process can also be traced in other ways, like by focusing on how the two sciences approach and understand thinking. Although thinking and language were not included in the research in this laboratory, Wundt (1897) regarded thinking as the core of psychology. As a commentary to Papanicolaou (Integr Psychol Behav Sci doi:10.1007/s12124-014-9273-3, 2014), this paper investigates the differences in how psychology and philosophy conceptualized thinking in early Western modernity. Thus one of the findings is that the separation process between the two was more or less initiated by Immanuel Kant. By defining thinking in terms of the pure reason he excluded the psychological understanding of thinking because psychology basically defined thinking in terms of ideas derived from qualia and sensation. Another finding is that psychology itself has not completely realized the differences between the philosophical and the psychological understanding of thinking by having been influenced by Kant's ideal of the pure reason. This may also explain some of the crises psychology went through during the twentieth century.