During the Pantheism controversy, they supported Kant and they believed that his intentions were noble, but they remained opposed to his critical philosophy. They recognized that Kant was trying to develop a synthesis between empiricism and rationalism, but they felt that he was biased towards rationalism. They held that his critical philosophy was dangerous because while intending to defend the authority of reason, it undermines it. During the 1780s and 1790s, they leveled against Kant the charge of Humean solipsism or nihilism. They accused him of being a dangerous skeptic and a dogmatic metaphysician.
The Lockean campaign against the Critique began with Christian Garve’s January 1782 review, which elicited from Kant an angry response in the form of the Prolegomena. In 1784 there was a review by Dietrich Tiedemann and an essay by C. G. Selle. In the same year, there was also a review of the Prolegomena by H. A. Pistorius. But by 1786, the Critique had become immensely popular and that caused even more nervousness in the Lockean circles, inspiring them to launch a new offensive. Kant was attacked in several reviews, essays and books.
Frederick C. Beiser, in his 1987 book The Fate of Reason: German Philosophy from Kant to Fichte offers an account of the Lockean campaign against Kant in chapter 6, “The Attack of the Lockeans.” Beiser says that as many authors were involved in the long drawn campaign, it is difficult to summarize the general arguments that the Lockeans used against Kant. But he offers seven themes that are characteristic of the Lockean campaign (and he also makes some points about the campaign against Kant by the Wolffians, the rationalist followers of Christian Wolff):
(1) One of the central issues between Kant and his empiricist opponents concerned the possibility of a priori knowledge. Every Lockean maintained that all synthetic knowledge is a posteriori, derived from and justified through experience. Some of them, however, were daring enough to argue that even analytic knowledge is a posteriori.
(2) Another basic conflict centered on the proper method of epistemology. The Lockeans advocated a purely naturalistic epistemology, that is, one which explains the origins and conditions of knowledge according to natural laws alone. Such an epistemology was obviously modeled upon the natural sciences; its prototype was "the plain historical method" of Locke's Essay or "the principles of observation and experiment" of Hume's Treatise. The Lockeans therefore rejected Kant's a priori method. They saw it as metaphysical and condemned it for forfeiting the ideal of a scientific epistemology.
(3) Yet another controversy surrounded the legitimacy of Kant's sharp dualism between reason and the senses, his radical dichotomy between the homo noumenon and homo phenomenon. The Lockeans regarded this distinction as arbitrary and artificial, as the reification of a purely intellectual distinction. Reason and sensibility were, in their view, inseparably united, different not in kind but only in degree. Of course, the Wolffians also attacked Kant's dualism; but there was still an important difference between the Lockeans and Wolffians on this score. While the Woffians saw sensibility as a confused form of the understanding, the Lockeans regarded the understanding as a derivative form of sensibility.
The Lockeans most often objected to Kant's dualism on the ground that it is antinaturalistic. It postulates a mysterious Platonic realm, the world of noumena, which is inexplicable according to natural laws. Kant's noumenal world makes the origin of our ideas and intentions obscure to us; and it renders the interchange between reason and sensibility unintelligible. Hence the Lockeans frequently accused Kant of 'mysticism', 'obscurantism', or ‘superstition'.
(4) The most notorious and controversial issue between Kant and the Lockeans concerned whether there is any essential difference between Kant's and Berkeley's idealism. Feder was the first to deny such a difference; and all the Lockeans, and most of the Wolffians, seconded him. The charge of Berkeleyan idealism was tantamount to the charge of solipsism, which was generally regarded as the reductio ad absurdum of the critical philosophy.
(5) The Lockeans were sharp critics of the "Aesthetik," and in particular Kant's theory that space and time are a priori. They argued that space and time are not a priori intuitions, but a posteriori concepts, which are abstracted from particular distances and intervals. Almost all of their early examinations of the Kritik focused upon the "Aesthetik," because it was seen as the test case for Kant's idealism and theory of the synthetic a priori. On the whole the Lockeans, like the Wolffians, ignored the "Analytik," passing it over in silence.
(6) The Lockeans criticized the way Kant classified concepts of the understanding as completely arbitrary and artificial. The Wolffians too made such objections to Kant. But the Lockeans, unlike the Wolffians, regarded any such classification as in principle mistaken. Maintaining that all concepts are abstractions from experience, they denied that there could ever be any complete list of all the possible concepts of the understanding.
(7) The Lockeans were the first to argue that the categorical imperative is empty, and that duty for duty's sake is in conflict with human nature. Against Kant, they defended eudaemonism as the only moral philosophy that can provide a sufficient criterion of morality and be in harmony with human needs.