In another letter, Külpe identified a key distinction in his ideas as compared to Wundt’s ideas. Külpe differentiates 'thought' from 'thinking'. Thoughts are 'contents’, he argues, whereas thinking in its various forms such as meaning, judging, and concluding can be considered acts or functions. In this case, the contents are the thoughts themselves, whereas the acts or functions are the thinking process. To Külpe, function and content are different and independently variable. He proposed that the acts or functions are not analyzable in consciousness, are relatively unstable, and can only be observed and known after an event has occurred. In order to observe them, Külpe’s process of systematic experimental introspection is critical. Külpe believed that contents, on the other hand, are analyzable in consciousness, observable in introspection, and relatively stable. He also cautioned against speculating about what is occurring physiologically at the time of thinking. Instead, Külpe encouraged the finding and improving of facts on the subject. That is, ignore the physiological processes occurring as part of the thinking process but instead introspect upon what you know has occurred.