Final answer:
Before the establishment of psychology as a science, philosophy was the primary field involved in exploring the mind. Notable philosophers and early scientific theories, like phrenology sought to understand mental processes before modern psychology's advent in the late 19th century.
Step-by-step explanation:
Prior to psychology, the field that was used for exploring the mind was primarily philosophy. Philosophers like Socrates, Plato, and Aristotle considered questions about the mind, existence, knowledge, values, reason, and language. During the 19th century, waxing philosophical about the mind transitioned towards a more empirical scientific approach with the work of thinkers such as Wilhelm Wundt and William James, who are credited with founding modern psychology as a formal science.
Speculative methods such as phrenology also attempted to understand mental processes, but these were later debunked. By questioning the existence of the soul and addressing the unconscious, figures like Sigmund Freud further complicated the issue, laying the groundwork for psychoanalytic theory. Eventually, the cognitive revolution re-centered psychology's focus onto the scientific study of the mind.