By Willard Hume
There is an important difference between accepting a theory at a theoretical level and accepting a theory at a practical level which is often not appreciated. The difference has to do with two distinct criteria for evaluating theories; truthfulness and usefulness. To evaluate a theory for truthfulness is to evaluate the theory on how successfully its contents, and the propositions entailed by its contents, capture truth. When we are evaluating for truthfulness, the best theories are those which make only true claims. To evaluate a theory for usefulness is to evaluate a theory in terms of how useful it is in helping us achieve some desired end. When we are evaluating for usefulness the best theories are the theories which are of greatest use.
Often, truthfulness and usefulness go hand in hand. That is, a truthful theory is often a useful theory (and vice versa). An example of this is scientific theories. When a scientific theory is better than its rivals at accounting for empirical evidence and making accurate predictions we usually judge it to be true (or at least we judge it to be the best approximation of the truth). But scientific theories which are true, or approximate the truth, also tend to be more useful because their predictive accuracy gives us a greater ability to anticipate nature.
Sometimes truthfulness and usefulness do not equate. In some cases a theory is useful but not truthful. For example it may be useful for somebody with low self-esteem to believe they are liked by their peers, even though this is false, because it will give them greater confidence. In other cases a theory is truthful but not useful. For example, it may be true that there are 7463 grains of rice in a rice bag and yet this may be of no use to anybody.
The distinction between truthfulness and usefulness in the evaluation of theories may seem fairly straight forward and of little philosophical significance. However, many plausible positions in a range of philosophical debates are overlooked because of a failure to take account of this distinction. For example, if you met someone who claimed to be a utilitarian and yet you observed them exclusively employing deontological principles in their moral decision making, you might conclude that they were inconsistent. But the distinction between truthfulness and usefulness shows us that this conclusion may not be right. For someone who accepts the truth of utilitarianism may also hold that following deontological moral principles is the most useful way to achieve the utilitarian end of the greatest happiness for the greatest number. In this respect they evaluate utilitarianism as the best theory in terms of truthfulness, and thus adopt the greatest happiness for the greatest number as their ultimate end, and yet also judge that following deontological principles is the best way to achieve this end and thus adopt deontology in their moral practice. Subsequently, their belief in utilitarianism at the theoretical level is entirely consistent with their adoption of deontology at the practical level.
This is just one example of how the distinction between truthfulness and usefulness can have philosophical traction. But there are many other philosophical topics where novel positions are available once this distinction is recognized.