From Dan Zahavi’s Husserl’s Phenomenology, paydirt:
The relation between the transcendental subject and the empirical subject is not a relation between two different subjects, but between two different self-apprehensions, a primary and a secondary. The transcendental subject is the subject in its primary constitutive function. The empirical subject is the same subject, but now apprehended and interpreted as an object in the world, that is, as a constituted and mundanized entity.
It is in this context that Husserl calls attention to the fact that subjectivity can be thematized in two radically different ways, namely in a natural or psychological reflection on the one hand, and in a pure or transcendental reflection on the other. When I perform a psychological reflection, I am interpreting the act reflected upon as a psychical process, that is, a process occurring in a psycho-physical entity that exists in the world. This type of self-consciousness — which Husserl occasionally calls a mundane self-consciousness — is just as worldly an experience as, say, the experience of physical objects, and if one asks whether it can provide us with an adequate understanding of subjectivity, the answer is no. Natural reflection presents us with a constituted, objectified, and naturalized subject, but it does not provide us with an access to the constituting, transcendental dimension of subjectivity.
It is here that the pure or transcendental reflection is introduced, since its specific task is to thematize a subjectivity stripped from all contingent and transcendent relations and interpretations.
Husserl makes it clear, however, that this type of reflection is not immediately available, so the question remains: What method or procedure can make it available?
The obvious answer: Through the epoché. For, as Husserl emphasizes again and again (with an obvious jab at introspectionism), unless the way has first been cleared by the epoché, we will be dealing with an objectified and mundanized experience regardless of how intensively or how carefully and attentively one reflects. In contrast to the positive sciences, which can proceed directly to their different fields of research, the region that phenomenology is supposed to investigate is not immediately accessible. Prior to any concrete investigation it is necessary to employ a certain methodological reflection to escape the natural attitude. Only through a methodical suspension of all transcendent preconceptions, only through a radical turn toward that which in a strict sense is given from a first-person perspective, can transcendental analysis commence.
No amount of introspection of the objectified psychological subject will give us insight into the transcendental subject. Efforts to make changes to the objectified psychological subject leave the world-constituting transcendental subject unchanged.
People will happily work on their psychological subject, as long as their transcendental subject is left undetected and sovereign. Any line of thought that makes the transcendental subject conspicuous or challenges its absolute authority will induce profound angst.