David Hume published his “Treatise on Human Nature” in 1739. And intellectuals all over the world have been touching base with his text ever since. That’s impressive. And what’s even more impressive is that he published the book when he was just 27 years old.
Perhaps the most provocative thing he said in those 368 pages is that you can’t move from “is” to “ought” by reason alone. The quote is fairly lengthy, so I’ll include it in a footnote. 
This short argument should illustrate the point:
Hume says, “Nope. Can’t do that.” Or rather, you can’t do it by reason alone. Reason can’t take you from “I weigh 250” to “I should go on a diet.”
The problem is that “I weigh 250” is a descriptive claim. It simply describes some state of the world.
And “I should go on a diet” is a normative claim. It prescribes action.
If you want to get to a normative conclusion using reason, you can’t do it from descriptive premises alone. You have to include some normative assumptions as well.
So we could argue this way:
And this would make Hume much happier, because the normative conclusion doesn’t follow from descriptive premises alone. There is at least one normative premise in the mix as well. 
OK, so what? Well, if you want to construct a semi-formal argument that will make Hume (or his followers) happy, then you will need to make the argument longer than you would if you were talking to a normal person. That’s what.
Actually, it turns out a lot of people take Hume’s strong distinction between is and ought seriously. Some seriously agree with it. And some seriously disagree with it. Many scientists consider themselves guardians of the gap between is and ought. And moral philosophers have heated debates about whether you can or can’t cross the gap with reason alone. 
But we’re not concerned with any of that here.
Our concern here today is not with norms of logic, but with practice. Our question is: “How do you get from is to ought (and back again) in practice?” In other words, can Hume’s sharp division between is and ought help us manage our personal growth? Can it help us overcome procrastination? Can it help us get more things done? And, if not, can it at least help us understand our struggles better?
It would seem so.
Before we see why, let’s clarify the situation. When we form a goal, we want to change things from how they are now to something new. In the abstract, it goes something like this:
Current state of things → Goal → New state of things.
And, since the current state of things is an “is,” the goal is an “ought,” and the new state of things will hopefully be another “is,” we need to cross the is/ought gap, not just once, but twice. In other words, we need to go from is to ought and back to is again. Schematically, it looks like this:
Is → ought → Is
And that invites a couple of questions. If reason can’t logically take us across these gaps, can it take us across them in practice? And if not, what can?
Many who went before Hume argued that we should ignore our passions and allow pure reason to be our guide. Hume’s response was this:
“Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.” (ToHN, book 2, part 3, section 3)
According to Hume, reason cannot motivate action on its own. Reason cannot take us from is to ought logically. And it can’t do it psychologically either. Reason cannot move us. It is there simply to serve the things that do move us—our passions.
Hume uses the word “passion” a little differently than we do when we announce: “I need to find my passion in life.” (Though the two are not completely unrelated either.) Hume’s “passion” is closer to our “desire” (and more specifically a desire to move away from things that are painful and toward things that are pleasant).
And which things turn out to be painful and pleasant will depend on our biological and psychological needs, such as food, shelter, avoiding injury, consensual sex, status, autonomy, competence, belonging, purpose, and companionship. If we have enough of these things, our lives are more pleasant and less painful. And, if we don’t have enough of these things, our lives are less pleasant and more painful. Not everyone has the same needs to the same extent. And psychologists will give different lists of basic human needs. But most lists tend to overlap with each other fairly substantially. (For more on human needs see: "What's Missing?" )
According to Hume, an ought will be motivating only if it aligns well with our wants and needs. If reason tells us to form a goal, and it goes against everything we want and need, we won’t be motivated at all to pursue the goal. And if we find ourselves pursuing a goal endorsed by reason, in spite of the fact that it runs counter to some of our needs, we should suspect that we are somehow serving other, deeper, needs.
If Hume is right, his dictum suggests the following:
The key to going from is to a motivating ought, then, on Hume’s model, is to manage the alignment of our goals with our needs.
If we take our needs as given, we should try to choose goals that fit them.
If we take our goals as given, then we should try to describe our current situation, our goal, and our imagined future, in a way that causes us to pay attention to the needs that are aligned with the goal, and ignore the needs that are frustrated by it. (Such trickery might or might not work, but it’s worth a shot, because it sometimes does, as in cognitive behavioral therapy.).
We can also do a little of each, manipulating the salience of the goal a bit, and changing the goal a bit, to move the goal and our passions closer together.
And, if we find a big enough goal, and it’s well enough aligned with our needs, I suppose we might be tempted to say that we have found our “passion in life”.
So we get from is to a motivating ought with passion. How then do we get from our motivating ought to the new “is” at which it aims?
Well, with action, of course.
We don’t get things done with reason alone. We get them done with passion and action.
Now here’s the thing about action: It always produces a new “is.” But it doesn’t always produce the “is” we are aiming at.
Just as our needs can be out of alignment with each other, and with our goal, our actions can be out of alignment with each other and our goal.
So the best way to get from “ought” to the “is” we want is with coordinated action. Our actions should be coordinated with each other, and with our goal.
And if all our needs are in alignment with a goal, and all of our actions are coordinated in serving that goal, watch out! That’s when stuff gets done.
Schematically, the full picture, in the ideal case, looks like this:
(IS) → (Aligned Passions) → (motivating OUGHT) → (Aligned actions) → (new IS)
So where does that leave reason? If all the work is done by passion and action, where does reason fit in? And, if reason is so unimportant, then why does it seem like those who are good at reasoning tend to get bigger and more impressive stuff done?
Hume doesn’t say that reason is not an important part of crossing the is/ought and the ought/is gaps. What he says is that it can’t do it on it’s own. And, in fact, it is a mere slave, and must serve the passions and actions that are the essential parts of getting things done.
We can get things done without reason. We do this when we are impulsive. Our needs suggest a goal, and action follows immediately. But impulsive action is often inferior to reason-aided action.
Here are some of the ways reason helps us form better goals and take better action:
Reason can help a lot with alignment issues. Without reason we will make only impulsive and uncoordinated lurches into the future. With reason we can take massive action (to quote Tony Robbins, who also sees the importance of aligning our goals with our needs and our actions with our goals) and impose our visions on the future.
If Hume is right about all this, and if there has to be a take-home lesson, I suppose the take-home lesson is this: If you find yourself procrastinating, don’t simply remind yourself of all the good reasons why you should work toward your goal. Take a moment instead to ask some deeper questions about your passions and your actions. Questions such as these:
There’s no guarantee that asking these questions will allow you to write a book that will still have people talking 378 years later. But it might help you get a few more important things done.
 Hume discusses the problem in book III, part I, section I of his book, A Treatise of Human Nature (1739): "In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason."
 Note that the premises might not all be true. The point here is that the inferences are valid
 Sam Harris is an example of someone writing on moral philosophy who does not observe the is/ought distinction. Simon Blackburn is an example of someone who observes it strictly.