If Philosophy

The Hypothetico-Deductive Method

Some knowledge is what we might call “analytic”. Say that someone tells you that the definition of things being lime coloured is that they have a “greenish yellow tint” or something similar. From this you can deduce that all lime coloured things are greenish. Sweet! Other things are what we come to know through experience in more than just an enabling sense. (Not sweet… We have to get up from our armchairs and explore the world? Oh, well.) Consider the ever so famous statement that the evening star is identical to the morning star. There is nothing within the definitions of these concepts that allows us to assent to the proposition before us. Even so, it is true. But how can we come to know about such a thing? We might come to know it through careful observation using some sort of telescope, for example. Such an observation would provide us with the justification to assent to this proposition. It does so in a different but analogous way to how understanding the concept of “lime coloured” and “greenish yellow tint” justified our assenting to the proposition that all lime coloured things are greenish. Empirical and analytic justifications are different, but serve similar purposes.

The evening star and other faraway objects make up one example of things not yet seen, though there are many others as well. The same underlying method can be used to provide justification for claims about things not yet seen. Some things are, however, unobservable. For instance, some claims concern general statements. (That is, claims that all so-and-so are thus-and-so.) Let us suppose that someone claims that ingesting liquorice roots increases the longevity of all mammalian animals. How would we go about finding out whether that is the case or not? We have one particularly useful method to employ; the hypothetico-deductive one.

So, what is it, and why is it effective? It is a method that consists of about five steps and ends up at a positive or negative outcome that either speaks in favour of a claim, or against it; though it doesn’t verify or disprove it entirely. It’s a useful tool then to start to justify claims of as yet unobserved or general things. It is hypothetic because we suppose some state of affairs could be the case, and it is deductive because we logically draw out observable consequences of what follows from such a supposition. Let us see how it works in (theoretical) action.

Asking the Question

First off, you begin by asking a question. In our case it might be something like: “what is the relationship between liquorice roots and the longevity of mammalian animals?”.

Formulating a Hypothesis

Secondly, you provide a tentative answer to your question; what we call a “hypothesis”. Perhaps that: “ingesting at least 5 g of liquorice roots per day increases the longevity of mammalian animals”.

Deriving Observable Consequences

Thirdly, you wish to have the ability to test this possible answer, so you deduce observable consequences from it. To do so, you often have to make use of what is called “auxiliary hypotheses”. These are claims that connect the main hypothesis to the observable consequence. You could think of auxiliary hypotheses as construing a bridge spanning across a chasm and two mountaintops. The first mountaintop is the possible answer, i.e. our hypothesis; whereas the second is a possible observable state of affairs that would be the case if the hypothesis were true. There are many such mountaintops, but any combination of a hypothesis, auxiliary hypotheses, and observable consequences allows us to formulate a test implication. In our case, the observable consequence might be that serving at least 5 g of liquorice roots to one sample will yield a higher longevity as measured in days compared to a control group who did not receive such a snack. An auxiliary hypothesis helping us establish this claim is that longevity is about “life-span as measured in days” (for example).

The test implication always takes the same form. It states that if our hypothesis is true, and our auxiliary hypotheses are true, then we should be able to observe something in particular. In our case it has to do with observing an increased longevity in our test group. Alright, those are some preliminaries out of the way. Time to test!

Moving from Concepts to Testing

The fourth aspect is to put this test implication into action. Now that we have it clearly formulated, we are ready to go observe whether or not our observable consequence actually obtains in the world.

Results of the Test

If the observable consequence obtains, then we might say that the world is compatible with the hypothesis being true. We can’t say that the hypothesis has been shown to be true, only that it has not yet been ruled out as a possible state of affairs. If, however, the observable consequence does not obtain, we can infer that the combination of hypothesis and auxiliary hypotheses do not hold true. This simply means that not all of the claims are true. Perhaps the hypothesis is true, but the auxiliary hypotheses (again, think of these as the bridge between the hypothesis and observable consequence we failed to observe) are faulty. Perhaps the hypothesis is wrong, or perhaps both. All we can be certain of is that not all of them are true, if we cannot obtain our expected observable consequence.


Some knowledge is what we might call analytical. To know that bachelors are unmarried men we do not have to do more than grasp the concepts involved. Other kinds of knowledge depend on experience in some way for justification. Some knowledge is justified by use of the hypothetico-deductive method. It consists of five steps, namely (i) asking a question, (ii) formulating a hypothesis, (iii) deriving observable consequences, (iv) testing, and (v) receiving a result.

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