As Brentano was very much concerned with criticizing positivism, he was compelled to pay particular attention to its most updated versions. The works of Ernst Mach were of special interest to him in this respect, for he regarded Mach’s philosophy as positivism in its most advanced form. His criticism of this version of positivism is moreover of great interest to us because Mach is an important link to the logical positivism that came to thrive in Vienna. In lectures, Brentano had argued against Mach’s allegedly anti-metaphysical doctrine of elements, the heart of his positivism, but after his retirement he wrote an extensive critique of Mach’s Erkenntnis und Irrtum (1905), which is the mature statement of the Machian orientation in philosophy. This contribution is an elaboration of Brentano’s critique in three different areas, namely, psychology, metaphysics, and logic.