Physicist and philosopher Ernst Mach investigates the physical sensations of sight, sound, motion, and the perceptions of space and time.

Mach is frank about his book incorporating both scientific and philosophical ideas. While acknowledging his investigations were on the border of what science had helped humans comprehend, he defends his work by stating that the manifold sensations a person experiences each day are the result of a complex interplay of biology and physics. As such, philosophical enquiries on the sensory experience are merited, for science alone cannot encompass or quantify the various sensations with a view to explaining their precise effects on the human mind and its thought processes.

An example investigated by Mach is that of time: how certain sensory experiences appear to make time pass slower or faster. Often vivid experiences, a shocking sight, produce the sensation of time slowing - why this occurs and how it affects the mind is discussed at length. Though Mach met with both praise and opposition with the successive editions of this work, he is cheerful at having aroused such debate; he had appropriated aspects of science to philosophic ends.