To begin with, recall that Kantian deontology is a form of deontology that places absolute moral value in the agent's intent. It doesn’t matter whether or not you succeed in bringing about what you intended.
Kant argued that an intention isn’t just any decision you make to do something. For Kant, intentions only move you to act in a certain way if they can be expressed as a kind of direction or command you give to yourself. Consider these examples.
If you carry out a maxim, then you've realized it in an act. Since Kant doesn’t think morality depends on whether or not you achieve your purpose, he doesn’t evaluate the action. Instead, it’s the maxim that’s to be evaluated.
To evaluate a maxim you need to see how it fits with the categorical imperative. If it violates the categorical imperative it’s impermissible (i.e. it’s wrong to do), if it doesn’t then it’s permissible (i.e. it isn’t wrong to do).
The idea of a categorical imperative is quite difficult to grasp. It’s fairly easy to think about a law or rule that applies to specific people.
But what does a demand or imperative look like when it’s not relative to the person following it? In other words, what are the rules or laws that everyone should be following?
Kant was aware of this difficulty, so he gave different formulations of the categorical imperative to make it easier to understand. One of the formulations is called the formulation of humanity. In short, it says you should always respect people’s humanity (including your own) when you make use of people for some purpose.
In order to get a better idea of what this means, let’s look at an example of not respecting humanity.
Imagine you have a friend called Habib, and you want to persuade him to not be friends with Olivia. You make up some reasons why Olivia wouldn’t be a good friend (e.g. she’s always running late).
If you succeed in making Habib not want to be friends with Olivia, then you would have abused his rationality by fooling him, so you can get him to do what you want.
In this example you would have deprived your friend of the opportunity to use his capacity to make a decision for himself. But if the reasons you gave were actually true, then you would be helping him to make an informed decision. You would still be using him as a means to your end (e.g. of making sure he doesn’t become friends with Olivia), but you would be respecting his humanity at the same time.
You might be wondering: is humanity really so important? Kant says it is because it’s needed for the good will, which is more important than anything else. The good will (i.e. the will that intends what’s right) is more valuable than, say, good talents or good fortune. That’s because, no matter how good your talents are, they aren’t always good.
Talent isn’t inherently valuable because its value depends on something else (i.e. the purpose you use it for). Only the good will has inherent value because only it doesn’t depend on something else for its goodness.
You couldn’t have a good will if you didn’t have humanity.
Since human capacities or humanity is needed for the good will, it's just as valuable as the good will. This is why Kant thinks you should respect humanity as well as the good will.
Now let’s try to apply this formulation. First of all, you need to identify a maxim. Then you need to see if that maxim treats humanity merely as a means, or whether it treats humanity as an end as well.
Your intention here would be to use your friend to help you understand what you’re studying. But since you do nothing to limit their rational or moral capacities, you also respect their humanity as an end. Therefore, it’s permissible.
Now let’s see what happens when we change this maxim.
In this case you don’t allow your friend to make the decision to help you for themselves. Instead, you make them do it by threatening to do something bad to them. So you don’t respect their humanity as an end in itself. Therefore, it’s impermissible.
The situation-specific principle of an action that an agent upholds by acting in that way