say that

say (that)

Suppose or assume that (some hypothetical were true). Say that you invest $200 into this account. With the interest rate they're offering, you would have $220 by the end of the year. OK, say this melon is the sun, this apple is Earth, and this marble is the moon. Now let's see how they all rotate in relation to one another.
See also: say
Farlex Dictionary of Idioms.

say that...

to assume [something]; to suppose [that something were so]. Say that x is equal to a whole number greater than 10. Say that two trains leave two different cities at the same time.
See also: say, that
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • say (that)
  • I suppose not
  • I dare say
  • get (someone or something) under (one's) control
  • I don't suppose you
  • I don't suppose you could...
  • assume the position
  • take (something) to be (something)
  • as long as
  • as/so long as
References in classic literature
I was sorry to hear Jim say that, it was such a lowering of him.
And I dare say that he did know, and that you know what he said: please, therefore, to remind me of what he said; or, if you would rather, tell me your own view; for I suspect that you and he think much alike.
SOCRATES: Then as he is not here, never mind him, and do you tell me: By the gods, Meno, be generous, and tell me what you say that virtue is; for I shall be truly delighted to find that I have been mistaken, and that you and Gorgias do really have this knowledge; although I have been just saying that I have never found anybody who had.
MENO: I should say that health is the same, both in man and woman.
I mean to say that strength, as strength, whether of man or woman, is the same.
SOCRATES: Then now that the sameness of all virtue has been proven, try and remember what you and Gorgias say that virtue is.
And if you answered 'roundness,' he would reply to you, in my way of speaking, by asking whether you would say that roundness is 'figure' or 'a figure;' and you would answer 'a figure.'
SOCRATES: And if he similarly asked what colour is, and you answered whiteness, and the questioner rejoined, Would you say that whiteness is colour or a colour?
Simonides, then, after the manner of poets, would seem to have spoken darkly of the nature of justice; for he really meant to say that justice is the giving to each man what is proper to him, and this he termed a debt.
And when you want to keep a shield or a lyre, and not to use them, you would say that justice is useful; but when you want to use them, then the art of the soldier or of the musician?
We should rather say that he is a friend who is, as well as seems, good; and that he who seems only, and is not good, only seems to be and is not a friend; and of an enemy the same may be said.
I believe that Periander or Perdiccas or Xerxes or Ismenias the Theban, or some other rich and mighty man, who had a great opinion of his own power, was the first to say that justice is `doing good to your friends and harm to your enemies.'
thof his skin be so white, and to be sure it is the most whitest that ever was seen, I am a Christian as well as he, and nobody can say that I am base born: my grandfather was a clergyman,[*] and would have been very angry, I believe, to have thought any of his family should have taken up with Molly Seagrim's dirty leavings."
I do know something of this lad, and in what I know of him, I can't say that there's any harm; perhaps on the contrary, constable." To whom the law-stationer relates his Joful and woful experience, suppressing the half-crown fact.
Chadband--of whom the persecutors say that it is no wonder he should go on for any length of time uttering such abominable nonsense, but that the wonder rather is that he should ever leave off, having once the audacity to begin--retires into private life until he invests a little capital of supper in the oil-trade.