It should be added, however, that, while, by turning the mind inward upon itself, we know what it is, we are, nevertheless, not able to define it. It is admitted, that it is not possible to give a definition of belief or faith, which, independently of inward experience, will render it easy to be understood. But this difficulty, whether it be regarded as greater or less, and which on a close examination will be found to be more formidable in appearance than in reality, is not limited to belief. All other states of mind, which are truly simple and undefinable, are better known by a reference to our own consciousness, than by any statements in words.
— edited from The Life of Faith (1852) Part 1, Chapter 1.