Analogy, a form of reasoning which argues that because objects are alike in many known particulars, they are also alike in some other and unknown particular. An illustration of an argument by analogy is conveniently drawn from the earth and Mars. Both are planets; they revolve about the same sun; they have the same shape; each has an atmosphere; each has a regular succession of seasons. There is an apparent similarity of rainfall, temperature, and the same succession of day and night. Now the earth sustains plant and animal life. Reasoning by analogy, it is urged that Mars is also clothed with vegetation and is inhabited by animals and people. It may be seen readily that analogy is likely to be exceedingly useful in stimulating investigation and in leading to discovery, but that it does not give absolute proof. The analogy seems to be perfect and complete, yet no one knows whether or not Mars is inhabited.