This paper is a commentary to Salvatore and colleagues (this volume) which discusses the foundation of a theoretical framework for psychology as a science. The paper argues that in general there are two fundamental sources for any theoretical frameworks in sciences, specifically philosophy and psychology. The argument is that psychology is historically the discipline that justifies an empirical approach in philosophy, whereas philosophy has traditionally only produced theoretical reasoning. This changed in the early modernity, in which philosophy and psychology became united. This unity produced different combinations of subjectivity and objectivity in philosophical reasoning. This paper presents synesthesia as a gateway to investigate the most rudimentary processing of a sense impression. From this perspective, the result demonstrates that the fundamental arbitrariness that forms intensional concepts is almost unavoidable.