In his early work (see, e.g., EPM §29), Sellars takes a hard line equating language possession and the ability to think, thereby denying thought to infralinguals, but in later pieces, most notably SK and MEV, he modifies this position.  He makes room for non-linguistic thought in humans, recognizing that visual perception and imagination are “in a sense most difficult to analyze, a thinking in color about colored objects” (SK I §37: 305).  Then, in “Mental Events,” Sellars introduces a distinction between propositional form and logical form that he did not utilize earlier.  “To have propositional form, a basic representational state must represent an object and represent it as of a certain character” (MEV: 336). Items in many non-linguistic representational systems such as maps possess propositional form by this characterization.  Logical form, however, is possessed by items that belong to representational systems that include distinct symbols for logical operators.  In this late essay, Sellars acknowledges that animals possess representations with propositional form and can make, correspondingly, what Sellars calls “Humean inferences,” that is, material inferences that employ no explicit generalization or logical principle.  His example is the move from “Smoke here” to “Fire nearby.” Human representational systems possess logical form; they enable us to engage in what he calls “Aristotelian inferences” in which a general principle is explicitly represented: “Smoke here.  Wherever there’s smoke, there’s fire.  So, Fire nearby.” This, Sellars claims, is the crucial difference between animal and human: full-fledged thinkers can recognize conditionality and generality as such, because they possess explicit symbols for if…then, all, and some.  