This inward-turned psychology stood in the service of the transcendental problem that had been awakened by Descartes, although this problem was not grasped in genuine form and properly formulated by Descartes himself. Still, in the very first of the Cartesian Meditations the thought was there – tangible, underdeveloped, but there and ready to be developed – a thought one can designate as the fundamental impulse of modern philosophy, that which essentially determines its particular style, namely: Every objectively real thing [alles Reale], and ultimately the whole world as it exists for us in such and such a way, only exists as an actual or possible cogitatum of our own cogitatio, as a possible experiential content of our own experiences; and in dealing with the content of our own life of thought and knowing, the best case being in myself, one may assume our own (intersubjective) operations for testing and proving as the preeminent form of evidentially grounded truth. Thus, for us, true being is a name for products of actual and possible cognitive operations, an accomplishment of cognition [Erkenntnisleistung].
Here lay the motivation for all the later transcendental problems, bogus as well as the genuine. Right away in Descartes the thought took a form which misled him and succeeding centuries. With seeming self-evidentness he proceeded in the following way: The experiencing and cognizing subjectivity is thrown upon its own resources. Cognition takes place within its own pure immanence. The evidentiality of the ego cogito, of pure subjective inner experience, necessarily precedes all other evidences, and in everything is already presupposed. How can I, the cognizing entity in this case, legitimately go beyond the component elements which are given with immediate evidentness to me alone? Obviously only through mediating inferences. What do these mediating inferences look like? What can give them that wonderful capacity to enter a world transcendent to consciousness? [p. 236]
The genuine transcendental problem is further obscured by the realism-problem, which misled centuries of thinkers with those absurd truisms [Selbstverständlichkeiten, self-evidentnesses] of a  theory based on inferences. All the same, the transcendental problem was prepared for and anticipated; attention was focussed on the all-embracing [universale] subjectivity of consciousness and its possession of a world. Descartes’ method of doubt can be designated as the first method of exhibiting transcendental subjectivity, at least that of the transcendental ego as a unified self centered in the ego and its cognitive life-process. One can say: it is the first transcendental theory and critique [in the Kantian sense] of universal experience of the world as the foundation for a transcendental theory and critique of objective science.
In unsuccessfully working out the transcendental problem, in the twisting involved in Descartes’ wrong formulation of the transcendental problem, the ego becomes pure mens [mind] as substantia cogitans [cognative substance], that is, mens as concrete mind [Seele] or animus, existing for itself yet again something that exists for itself only through causal law and its link with corporeal substance.
Locke, without sensing the depths opened up by the first Meditations and the fully new position attained there in relation to world and to mind, took the pure ego from the outset as pure mind-substance [reine Seele], as the “human mind,” whose systematic and concrete exploration on the basis of evident inner experience was to be the means of solving the questions of understanding and reason. However great his epoch-making contribution was, of having posed this question concretely and in the unity of a scientific-theoretical horizon and of having shown its relationship to the primal foundation in inner experience, still he missed its genuine transcendental meaning because he conceived of it as psychological inner experience.
So he became the founder of psychologism, a science of reason – or as we can also say it in a more general way: a transcendental philosophy on the foundation of a psychology of inner experience.
The destiny of scientific philosophy hinged, and still hinges, on establishing it as genuine transcendental philosophy, or what goes with this, on a radical overcoming of every form of psychologism; a radical overcoming – namely one that lays bare in one stroke what is sense, what is in principle nonsense, and yet what is its transcendentally significant kernel of truth. The source of psychologism’s continuous and  invincible power through the centuries comes, as will be shown, from drawing on an essential double meaning which the idea of subjectivity and therewith all concepts of the subjective take on, and which arises as soon as the genuine transcendental question is posed. The disclosure of this double sense which links psychological and transcendental subjectivity together, and indeed not accidentally unites them, is brought about when the divorce is accomplished between phenomenological psychology and [p. 237] transcendental phenomenology – one as rational psychological foundational science and the other as rational foundational science of philosophy in its necessary form as transcendental philosophy. In connection with this, the idea also seems to be justified of phenomenological psychology being projected as an advance guard for and valued as a means of access to transcendental phenomenology. [p. 238]
Edmund Husserl, Psychological and Transcendental Phenomenology and the Confrontation with Heidegger 1927-1931, pp. 236-238