intelligence means a truth which is valid for all intelligences which may have existed in the countless ages of the past, or which may exist in the countless ages of the future. Now, I am under no compulsion to think that the earth from all eternity has revolved around the sun, or that it will continue throughout all eternity so to revolve around the sun; in other words, I can help thinking that it always has travelled, and that it always will travel, as it now travels. I can conceive the operations of the universe changed. This, therefore, is not a truth valid at all times for all intelligence. Take another case. I say, The square on the hypotenuse of a right-angled triangle is equal to the squares on the other two sides; or, to take a simpler case, I say that two straight lines cannot enclose a space. Are these truths which exist for all intelligence? Yes, they undoubtedly are. Take the former: it is a truth which is valid for all intelligence. And why do I so regard it? Simply because I am compelled. I cannot help thinking it as a truth which every intelligence which follows the demonstration must assent to. And why can I not help thinking it to be a truth of this character? Because I cannot conceive that any difference in the organism, or any difference in the constitution of the universe, or any difference in the intelligence which apprehends it, should cause it to be apprehended differently. I cannot conceive any mind which understands the demonstration to hold that the squares on the two sides are either greater or less than the square on the third
