Learn English – Past tense of “must” when meaning logical probability

As already properly commented by aedia, the word contained in the example you presented is not the past tense of find, but rather a different word, to found, meaning to establish.

I'm sure you've heard it used before, in examples like:

This company was founded in 1950.

That's the past tense. The infinitive is used in your example:

We can found no scientific discipline.

Which can be rewritten to:

We can establish no scientific discipline.

Rarely, must is used as a past tense. Belshazzar, by H. Rider Haggard, has we went because we must, in a prose style which is perhaps deliberately archaic to reflect the ancient Egyptian context.

In this odd snippet, If Thoreau went because he would, Hawthorne went because he must, one might say the author is "playing with language".

But here's Ralph Waldo Emerson with What he did, he did because he must. I would not wish to say Emerson doesn't know his own language.

From comments under @Henry's answer, it seems something quite odd has been going on. Many people will know the archaic present tense mote because Freemasons & such still say So mote it be in a "ritual" context. Bizarrely, the past tense "must" eclipsed "mote" for present tense usage. But in so doing, "must" somehow almost completely lost its ability to still be used as a past tense.

In spite of all the above, ordinary mortals in ordinary contexts today should stick with the standard position put forward by other answers. Use had to for the past tense.