Received and already skimmed the introductory chapters (and index and references) of Ernst Mach – The Analysis of Sensations, and the Relation of the Physical to the Psychical. (1914 translation of the 1906 5th edition of the 1885 original), and – Space and Geometry, in the Light of Physiological, Psychological and Physical Inquiry. (1906 edition, 1901, 2 and 3 originals)
As well as many psychology references to William James, many “antimetaphysical” philosophy references, that make promising reading:
People new to me but apparently important to Mach like Rudolf Willy and Richard Avenarius (the latter – started with Spinoza).
A full enumeration of Mach’s philosophical position would require us to begin with Spinoza.
Obviously – not essentially different from Hume.
Borders closely – the immanence of Schup.
Psychological facts at least as important as the physical facts – contrary to Comte.
Affinity with Avenarius, so great as can possibly be imagined yet having followed entirely different histories.
Plato’s fiction – the unfortunate antithesis between appearance and reality in The Cave – not thought out – lasting influence needing correction to put everything right again. (Cf Barfield Saving the Appearances)
So, Mach’s starting point :
The physiology of the senses demonstrates that
spaces and times may be just as appropriately
called sensations as colours and sounds.