Mathematical writing: why should we not use the phrase "we have that"?

Solution 1:

This is absolutely not a matter of English usage, but a matter of mathematical usage. I once said to a colleague of mine in the Anthropology Department that mathematicians often say, and write, “We have that $A=B$”, and his mouth dropped wide open. Now I have to admit that the Great and Blameless Serge Lang often wrote the offending words, but after all he was not a native speaker of English, and I think it’s wrong, wrong, wrong. Here’s my analysis of what’s going on:

The conjunction that is used to introduce a dependent noun clause of indirect statement: “I know that my Redeemer liveth”, “I’ve heard that you were sick.” My firm conviction is that such a clause can be introduced by, can be the object of, only verbs of sensing, thinking, or saying. Maybe a few other kinds of verbs. But it can’t be the object of the verb “have”. Syntactically it’s fine, “have” takes a noun as object, the clause of indirect statement is a noun clause, it all fits. It’s just that sensitive native speakers of English don’t complete that verb with that kind of clause.

And I agree with Knuth, that it’s perfectly all right to say “We have $A=B$.” Here the equation as written stands as a thing-in-itself, maybe you can explain the construction as a short form of “We have the relation $A=B$.” So the object of “have” is either the whole equation “$A=B$” or the noun “relation”, with the equation standing in apposition to the noun.

One more remark, before I get off my high horse: When refereeing a paper, I always say that I strongly urge the author to change the offending construction, but I do not insist. The usage is too well established to try singlehandedly to change things. But it’s still wrong, wrong, wrong.

Here endeth the sermon.

Solution 2:

"We have that" is just poor use of language. "We know that" or "We see that" or "We have assumed that" all sound much more natural.

Edit: I won't claim never to have used the construction, but I don't like reading it and I'd take pains to avoid it in a formal setting.

Solution 3:

Personally, I'm all for parsing sentences, and agree with the first remark made by Knuth. But I disagree with Knuth regarding his caution/exception. I think there are times when using the phrase "we have that" can be used to help parse sentences and enhance the clarity of an argument or proof.

So it seems to me that Knuth is voicing his preference regarding the phrase "we have that", and that is all well and good. But I think he went a little "overboard" in warning readers to never say it. There is nothing very wrong with saying "we have that $x = y$" or "we have that $x \gt y$". I find it quite natural to say, read, and hear it. I would prefer that Knuth save his vehement objection to the phrase to address more mathematically significant issues.

(Admittedly, I am writing this, in part, in defense of my rather widespread use of the phrase!)

Solution 4:

I regard the form “We have that X” as an elliptical form of “Thus, we have established that X...” or “Thus, we have proved X”. In other words, it emphasizes that it has been shown that X holds. “We have X” may say the same thing, but does not spell out if X has been proven or merely supposed.

Because the form sounds and appears slightly clumsy, Knuth is within his rights to inveigh against it. On the other hand, the problem does not seem so serious that people should lose sleep over it. It is a style issue moreso than a grammar issue.