  Russell maintained that an agent must be acquainted with every constituent of a thought that she is in a position to entertain. Call this the Acquaintance Principle. (The principle makes an appearance in (1905), but the view behind the principle is worked out in (1910) and (1912).) Russell thought that we are acquainted only with our occurrent sense data and universals (and he sometimes included the self, when he wasn't feeling skeptical of awareness of the self as an entity). So, Russell maintained that the only singular propositions we can grasp are ones with those items as constituents.