We know that Mercury is three times nearer that vast body of Light than we are. Whence it follows that they see him three times bigger, and feel him nine times hotter than we do. Such a degree of Heat would be intolerable to us, and set afire all our dry’d Herbs, our Hay and Straw that we use. And yet I warrant the Animals there, are made of such a temper, as to be but moderately warm, and the Plants such as to be able to endure the Heat. The Inhabitants of Mercury, it’s likely, have the same opinion of us that we have of Saturn, that we must be intolerably cold, and have little or no Light, we are so far from the Sun. There’s reason to doubt, whether the Mercurians, tho they live so much nearer the Sun, the Fountain of Life and Vigour, are much more airy and ingenious than we.--From Cosmotheoros--
This is nothing next to his reasoning that aliens must have hands so they can write down their astronomical observations. But that's metaphysics for you.