Type-identity is the most obvious way to ensure the non-separateness of special and physical causes: if exactly the same properties comprise the special and physical cause, the two causes will themselves be fully identical. Still, type-identity is a very strong doctrine. Type identity about thoughts, for example, would imply that the property of thinking about the square root of two is identical with some physical property. And this seems highly implausible. Even if all human beings with this thought must be distinguished by some common physical property of their brains—which itself seems unlikely—there remains the argument that other life-forms, or intelligent androids, will also be able to think about the square root of two, even though their brains may share no significant physical properties with ours (Fodor 1974, Bickle 2013).