We get the information (truths assumed in the premises of an argument) from observing the world around us and then drawing general conclusions from those observations. When you observe something over and over again, you may conclude that some general principle is true. For example, when you repeatedly drop an object off a table, you naturally observe that the object always falls to the floor. If you do that enough, you finally realize that there must be some general principle in place known as gravity. This method of drawing general conclusions from specific observations is called induction (which is commonly equated with the scientific method). In order to be clear, we need to distinguish induction from deduction. The process of lining up premises in an argument and arriving at a valid conclusion is called deduction.
That's what we did in the arguments above. But the process of discovering whether the premises in an argument are true usually requires induction. Much of what you know, you know by induction. In fact, you've already used induction intuitively to investigate the truth of the premises in the arguments above. Namely, you determined that since every man you've observed has been a two-legged mammal, the man John can not be a four-legged reptile. You did the same thing with the question of Spencer's mortality. Since all men you've heard about ultimately die, you made the general conclusion that all men are mortal including a specific individual man named Spencer. These conclusions-two-legged men, gravity, and human mortality-are all inductive conclusions.
Most conclusions based on induction cannot be considered absolutely certain but only highly probable. For example, are you absolutely, 100 percent certain that gravity makes all objects drop? No, because you haven't observed all objects being dropped. Likewise, are you absolutely certain that all men are mortal? No, because you haven't observed all men die. Perhaps there's someone somewhere who hasn't died or will not die in the future.
So if inductive conclusions are not certain, can they be trusted? Yes, but to varying degrees of certainty. As we have said before, since no human being possesses infinite knowledge, most of our inductive conclusions can be wrong. (There is one important exception. It's called the "perfect induction," where all the particulars are known. For example, "all the letters on this page are black." This perfect induction yields certainty about the conclusion because you can observe and verify that every letter is indeed black.)
But even when we don't have complete or perfect information, we often have enough information to make reasonably certain conclusions on most questions in life. For example, since virtually everyone has been observed to die, your conclusion that all men are mortal is considered true beyond a reasonable doubt; it's 99 plus percent sure, but it's not beyond any doubt. It takes some faith-albeit a very small amount-to believe it. The same can be said for concluding that gravity affects all objects, not just some. The conclusion is practically certain but not absolutely certain. In other words, we can be sure beyond a reasonable doubt, but not sure beyond all doubt.
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