What do you think that you know, and how do you know that it's so?

Did you hear from a friend or go take a fresh look?
Did you prove it with math, or just learn from a book?
There are things we believe in our hearts cause we're told
by a person which we trust, quite wise and quite old.

It is good to have thoughts where you reason about
what you're right to have trust in and what to still doubt.
For a thought can grow stronger when tested by fate
if you look, and you learn, and you're quick to update.

You can know any fact and can think any thought.
You can check it yourself. You can and you ought.
For a thing isn't true just because it's been taught.
You must test what you think and admit if it's not.

Now there's an odd beast that they call a Snaw.
It will spit any wrong falsehood straight from its maw.
Snaws give guesses and inklings the weight of a law
but can't tell what they think from what a snaw saw.

For a Snaw only feels that a thing must be true,
then they shout it so loud that the crowd thinks so too.
They will skip all the checks, and the facts, and the test.
For a Snaw calls it truth when it’s really just guessed.

For a Scientist checks things again and again
so that others can follow the path where they've been.
For they can get it right, or may make a mistake.
So the best of their work is the kind that’ll replicate.

On a lake where the fish often jumped and fish swam
some old Snaw and a scientist's boats one day bumped.
"Now get out of my way!" that old Snaw loudly said
"My boat's faster than yours, I can go on ahead!"

"That is strange," came the claim from the Scientist's view,
"for it looked like just now I was faster than you."
"I have made my own boat, I have built it for speed,
so now how do you know that you're faster indeed?"

“I feel it!” said the Snaw. “I can feel that I’m right!”
The Scientist smiled, “Then test it tonight.
If it works when we measure, I’ll gladly agree.”
Then the Snaw shouted loud, “No need! It just seems so to me!”

"Why test?" said the Snaw with a sniff.
I feel it, I know it, no need for your 'if.'"
The Scientist said, “There’s a way we can see.
Just try it and prove what you feel ought to be.”

The sun shone so bright, but the Snaw hid his nose,
“if we test,” he said softly, “who knows how it goes?
What if I am slower? I’d rather not see.”
The Scientist answered, “That’s a sad way to be.”

“Because,” said the Scientist, “feelings can lie.
The waves can be tricky, the wind can go by.
If you race and record it you'll have surety.”
The Snaw thought a while. “Well, I guess we can see.”

They lined up their boats at the mouth of the bay,
then off through the ripples they splashed, no delay!
The Snaw started strong with a whoop and a cheer.
But the Scientist’s boat kept on pulling up near.

They paddled together, the waves keeping score,
each gust gave a challenge, each splash one test more.
The Snaw’s boat fell back as the truth came in view.
That feeling alone wasn’t proof of what’s true.

The Snaw gave a sniff and looked down at his oar,
“Perhaps I’ll test twice before shouting ‘I’m sure!’”
Then the Scientist smiled, “That’s a start. If you might be beat,
it is wise to leave room for a line of retreat.”

For your rational knowledge is simple and neat. 
It is how your own mind keeps its balance and beat.
When you gather new data, that changes the score,
so your confidence shifts just a bit more and more.

First you start with the odds that your story is true,
then you weight those old odds by the strength of the clue;
Then compare with the chance you’d see that if it’s fake,
and those weighted new odds tell what stance you should take.

Oh, the places you’ll go! When you’re careful to know!
What you think that you know, and how you know so.