According to Kant, the fundamental principle of morality must be a categorical, rather than a hypothetical imperative, because an imperative based on reason alone is one that is a necessary truth, is a priori, and is one that applies to us because we are rational beings capable of fulfilling our moral obligations. Kant explains this essential truth is how "an action as objectively necessary in itself apart from its relation to a further end". This refers to how if the supreme principle of morality was only a possible truth, then its force as a principle would be dependent on what may happen in other propositions. So being, the SPM would not be guiding the choice and action of a morally good will. This means that because morality holds
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Kant divides hypothetical imperatives into two subcategories: the rules of skill and the counsels of prudence. The rules of skill are conditional and are specific to each and every person to such that the skill is mandated by experience (or, a posteriori knowledge), while, the counsels of prudence are attained a priori. But even in these two subcategories, it should be noted that the distinction between categorical and hypothetical imperatives is typically said to be that only the latter depend on our desires. This is misleading. Certain categorical imperatives may also be conditional on our desires, such as "if you want to torture children, you should seek psychiatric help." Of course, the required course of action is not merely a means to fulfilling the antecedent desire. But we could imagine this as a scenario that we're categorically required to act so as to fulfill our present desires, whatever they may be. So how are hypothetical imperatives different then?
One difference is that categorical (not hypothetical) imperatives allow for modus ponens, or detachment of the consequent obligation. This could be seen in the following: If you want to torture children, you should seek psychiatric help. You want to torture children. Therefore, you should seek psychiatric help. While this seems perfectly valid, suppose we replace the first premise with a hypothetical imperative: If you want to torture children, you should volunteer as a babysitter. You want to torture