What in effect characterizes empirical psychology is not in the first instance its preference for external knowledge, but its reduction of acts (with their intentionality and their reference to an Ego) to facts. Could we say that acts are better known from the "inside" and facts from the "outside"? That is only partly true.
For introspection itself can be degraded to a knowledge of facts if it omits the mental as intentional act and as someone's act. That is what happened to empirical interpretation of introspection in Hume and Condillac.
Introspection can be interpreted in a naturalistic sense if it translates acts into the language of anonymous facts, homogeneous with other natural facts, that "there are" sensations as "there are" atoms. Empiricism is a discourse in the mode of "there is." Inversely, knowledge of subjectivity cannot be reduced to introspection just as empirical psychology cannot be reduced to a psychology of behavior. Its essence is to respect the originality of the Cogito as a cluster of the subject's intentional acts. But the subject is myself and yourself.
These remarks are decisive for understanding the concept of a personal body. A personal body is someone's body, a subject's body, my body, and your body. For while introspection can be naturalized, external knowledge can in turn be personalized. Empathy (Einfuehlung) is precisely the reading of the body of another as indicating acts which have a subjective aim and origin. Thus subjectivity is both "internal" and "external." It is the subject function of someone's acts. By communicating with another, I have a different relation to a body which is neither included in perception of my own body, nor inserted in an empirical acquaintance with the world.
I discover body in the second person, body as motive, organ, and nature of another person. I read decision, effort, and consent in it. It is then not an empirical object, a thing. The concepts of subjectivity (of the voluntary and the involuntary) are formed by gathering experience derived from multiple subjects. On the one hand, my consciousness is profoundly transformed by the reoccurrence of the other's consciousness in it. I treat myself as a you which in its external appearance is a presentation to the other. From this viewpoint, to know myself is to anticipate my presentation to a you.
On the other hand, knowledge of myself is always to some extent the guide for deciphering the other, even if the other is in the first place and principally an original revelation of empathy. The you is an other myself. Thus the concepts of subjectivity, valid directly for my fellow man and going beyond the sphere of my subjectivity, are formed in mutual contact of reflection and introspection.
We can see from this that the transition from the phenomenological to the naturalistic viewpoint does not take place by inversion of the internal and the external, but by a degradation of both. My body is displaced from my subjective realm, but likewise your body is displaced from its subjective expression. The inert and inexpressive body has become an object of science. The object body is the body of the other as well as my body severed from the subject which each affects and expresses. Thus we can go from an object body to a subject body only by a leap which goes beyond the order of things, just as we move from the latter to the former by diminution and suppression, such diminution and such suppression being made legitimate by the type of interest represented by the constitution of empirical science as a science of facts.
Paul Ricoeur, Freedom and Nature: The Voluntary and the Involuntary, translated by Erazim V. Kohák