I. Haeckel and His Opponents

This fact is incompatible with the dualistic world-conception. The point which the supporters of this conception so often emphasise, namely, that the manifestations of the thinking consciousness are accessible to us through the inner sense of introspection, while we only comprehend physical and chemical happenings when we bring into the appropriate connections the facts of observation through logical, mathematical combination, and so on; in other words, through the results of the psychological domain: this fact is the very thing which they should never admit. For let us for once draw the right conclusion from the knowledge that observation transforms itself into self-observation when we ascend from the scientific into the psychological domain.