One of the core principles -- maybe the most core principle -- of the art of rationality is that beliefs are for true things. In other words, you should believe things because they are true. You should not believe things that are not true.
Holding that beliefs are for true things means that you do not believe things because they are useful, believe things because they sound nice, or believe things because you prefer them to be true. You believe things that are true (or at least that you believe to be true, which is often the best we can get!).
Eliezer referred to this principle as "the void", writing in his "The Twelve Virtues of Rationality":
Before these eleven virtues is a virtue which is nameless.
Miyamoto Musashi wrote, in The Book of Five Rings:
“The primary thing when you take a sword in your hands is your intention to cut the enemy, whatever the means. Whenever you parry, hit, spring, strike or touch the enemy’s cutting sword, you must cut the enemy in the same movement. It is essential to attain this. If you think only of hitting, springing, striking or touching the enemy, you will not be able actually to cut him. More than anything, you must be thinking of carrying your movement through to cutting him.”
Every step of your reasoning must cut through to the correct answer in the same movement. More than anything, you must think of carrying your map through to reflecting the territory.
Musashi wrote that you must always think of carrying your motion through to cutting; I write, with Eliezer, that every belief and every step in your belief must cut through to knowing the truth.
Beliefs, after all, are for true things, and if you lose sight of that you will lose your epistemics. If you think only of what gives you an advantage in a debate, of what sounds nice, of what wins you the admiration of your peers, of what is politically correct, or of what you would prefer to be true, you will not be able to actually believe true things.
I would like to take the perhaps unusual step of closing with a poem by Rudyard Kipling, which addresses this point (among others) rather well:
As I pass through my incarnations in every age and race,
I make my proper prostrations to the Gods of the Market-Place.
Peering through reverent fingers I watch them flourish and fall.
And the Gods of the Copybook Headings, I notice, outlast them all.
We were living in trees when they met us. They showed us each in turn,
That water would certainly wet us, as Fire would certainly burn:
But we found them lacking in Uplift, Vision, and Breadth of Mind,
So we left them to teach the Gorillas while we followed the March of Mankind.
We moved as the Spirit listed. They never altered their pace,
Being neither cloud nor wind-borne like the Gods of the Market-Place;
But they always caught up with our progress, and presently word would come
That a tribe had been wiped off its icefield, or the lights had gone out in Rome.
With the Hopes that our World is built on they were utterly out of touch.
They denied that the Moon was Stilton; they denied she was even Dutch.
They denied that Wishes were Horses; they denied that a Pig had Wings.
So we worshiped the Gods of the Market Who promised these beautiful things.
When the Cambrian measures were forming, They promised perpetual peace.
They swore, if we gave them our weapons, that the wars of the tribes would cease.
But when we disarmed They sold us and delivered us bound to our foe,
And the Gods of the Copybook Headings said: "Stick to the Devil you know."
On the first Feminian Sandstones we were promised the Fuller Life
(Which started by loving our neighbor and ended by loving his wife)
Till our women had no more children and the men lost reason and faith,
And the Gods of the Copybook Headings said: "The Wages of Sin is Death."
In the Carboniferous Epoch we were promised abundance for all,
By robbing selective Peter to pay for collective Paul;
But, though we had plenty of money, there was nothing our money could buy,
And the Gods of the Copybook Headings said: "If you don't work you die."
Then the Gods of the Market tumbled, and their smooth-tongued wizards withdrew,
And the hearts of the meanest were humbled and began to believe it was true
That All is not Gold that Glitters, and Two and Two make Four —
And the Gods of the Copybook Headings limped up to explain it once more.
* * * * * *
As it will be in the future, it was at the birth of Man —
There are only four things certain since Social Progress began: —
That the Dog returns to his Vomit and the Sow returns to her mire,
And the burnt Fool's bandaged finger goes wabbling back to the Fire;
And that after this is accomplished, and the brave new world begins
When all men are paid for existing and no man must pay for his sins,
As surely as Water will wet us, as surely as Fire will burn,
The Gods of the Copybook Headings with terror and slaughter return!