Genesis 15:5

And he brought him forth abroad
Out of his tent into the open air, which was done through his call, and at his direction; or by an impulse upon his mind; or this might not be real and local, only vision:

and said, look now towards heaven;
either with his bodily eyes, or with the eyes of his mind:

and tell the stars, if thou be able to number them;
this looks as if it were in a vision that this was said to him, and what follows done in the day, since it was in the daytime, before the sun was set, ( Genesis 15:12 ) , when the stars could not be seen; and therefore were represented to his mind, and he was directed to consider them in it, whether they could be numbered by him or not: but this might be in the preceding night, or early in the morning, before the sun arose, that Abram was directed to go out of his tent, and view the heavens, and the multitude of stars in them, and try if he could number them; and he might be employed all the day following till sunset, in preparing the creatures for the sacrifice, in cutting them asunder, laying their pieces in order, and watching them, and driving the fowls from them. The multitude of his seed is before signified by the dust of the earth, which cannot be numbered, ( Genesis 13:16 ) , and here by the stars of the sky innumerable; as they are to man, though not to God: some have pretended to number them, as Aratus, Eudoxus, and Hipparchus, among the ancients, and also modern astronomers; but then they are such only that are visible to the eye, and in one hemisphere, and their accounts are very various; whereas there are multitudes to be discerned by glasses, and some not to be distinguished, as in the galaxy, or milky way, and others in the other hemisphere. Now Abram here is bid to try what he could do, and this was in his own way; for he is said by many Heathen writers ^F8 to be famous for arithmetic and astrology, or astronomy; but as great a master as he was in these sciences, be was not able to number the stars, which is here plainly intimated, since it follows: and he said, so shall thy seed be:
as innumerable as the stars, as they were, even his natural seed, ( Hebrews 11:12 ) ; and especially his spiritual seed, who have the same kind of faith he had, and as they will be in the latter day particularly, ( Hosea 1:10 Hosea 1:11 ) .