This Chapter may be characterized as a parallel between the difficulties alleged to be fatal against Theism, and the difficulties attaching to very various departments of human knowledge, embracing its most necessary and its most certainly accepted kinds. From this parallel the conclusion becomes evident, that whoever accepts one set of truths cannot be debarred by these or similar difficulties from accepting the higher truth likewise. That such an acceptance is natural and valid appears further evident from the fact that a knowledge of God belongs to the class of Practical beliefs, and is enforced by the same reasonable necessity. This topic forms the transition to Chapter IV. on "Our Reasonable Beliefs."
The same inferences are also stated in a destructive form, e.g., Should a thinker choose to deny the possibility of Theism, he ought (if consistent) to deny all those truths which stand or fall by a parallel set of reasonings. But by doing this he lands himself in a state of doubt, so extreme and thorough, that the whole Universe becomes a rayless blank.
A corollary is added on Materialism.
Analysis—Man the interpreter of Nature. Nature gives by answering our interrogations; these must depend on our powers of assimilating knowledge. Some questions inevitable, e.g., What are the first grounds of Truth?
Has Man any faculty of apprehending the Infinite? Can we know our own Personality or that of others?—or any Thing in itself? Inference against Scepticism based on human ignorance.
Fallacy of the Unthinkable or Inconceivable. Ideas of Self and not-Self, inexplicable, yet undoubted. From things as they are, let us turn to things as they appear. How do we perceive, hear, see?
Perception as an instrument of Intelligence, inscrutable. We acknowledge the insoluble mystery but accept the fact.
Marvels of eyesight, and their problems. How much and what do we see? Comparison with Sound;—Form, Colour, Tone. Evidence on which we receive sense impressions. Comparison between healthy and diseased sensations,—between our organs of sense and those of animals. We soon arrive at a twilight territory of knowledge and can explain no more.
Imperfections in our powers of Verification. How great is the subjective Element in our perceptions? Idealism,—most difficult to answer when most extreme. Philosophic denial of all proof of external things as distinguished from Mind (e.g., by Mill). Fact-knowledge, and absurdities involved in the ordinary method of defining and alleging Facts. Polar tendencies of Phenomenalism which take the shapes of Idealism and Positivism, resulting in Nihilism or Indifferentism. The end of these things! Mr. Herbert Spencer on Theology, compared with Mr. Huxley, and criticized by Mr. J. Martineau, who denies that the Unknowable can be any object of religious feeling,—a protest strongly maintained by Mr. J. S. Mill.