The principles of science : a treatise on logic and scientific method / by W. Stanley Jevons.

  • Jevons, William Stanley, 1835-1882.
Date:
1874
    No text description is available for this image
    Laws of Identity and Difference. At the basis of all thought and science must lie the laws which express the very nature and conditions of the discriminating and identifying powers of mind. These are the so-called Fundamental Laws of Thought, usually stated as follows :— 1. The Law of Identity. Whatever is, is. 2. The Law of Contradiction. A thing cannot both be and not be. ± 3. The Law of Duality. A thing must either be or not be. The first of these statements may perhaps be regarded as a description of identity itself, if so fundamental a notion can admit of description. A thing at any moment is perfectly identical with itself, and if any person were unaware of the meaning of the word ' identity' we could not better describe it than by such an example. The second law points out that contradictory attri butes can never be joined together. The same object may vary in its different parts; here it may be black, and there white; at one time it may be hard and at another time soft: but at the same time and place an attribute cannot be both present and absent. Aristotle truly described this law as the first of all axioms0—one of which we need not seek for any demonstration. All truths cannot be proved, otherwise there would be an endless chain of demonstration; and it is in self-evident truths like this that we find the fittest foundation. The third of these laws completes the other two. It asserts that at every step there are two possible alter natives—presence or absence, affirmation or negation. Hence I propose to name this law the Law of Duality,
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image