Hermann Lotze argued that the fact that consciousness simultaneously “holds objects together as well as apart” such that they can be compared implies (a) that there is a simple thinker and (b) that consciousness is an ‘indivisible unity.’ I offer a reconstruction and evaluation of Lotze’s Argument from Comparison. I contend that it does not deliver (a) but makes a good case for (b). I will relate Lotze’s argument to the contemporary debate between “top-down” and “bottom-up” views of the unity of consciousness and locate it in its historical context. (Kant and Herbart figure prominently here.)

Bolzano incorporated Kant's distinction between intuitions and concepts into the doctrine of propositions by distinguishing between conceptual (Begriffssätze an sich) and intuitive propositions (Anschauungssätze an sich). An intuitive proposition contains at least one objective intuition, that is, a simple idea that represents exactly one object; a conceptual proposition contains no objective intuition. After Bolzano, philosophers dispensed with the distinction between conceptual and intuitive propositions. So why did Bolzano attach philosophical importance to it? I will argue that, ultimately, the value of the distinction lies in the fact that conceptual and intuitive truths have different objective grounds: if a conceptual truth is grounded at all, its ground is a conceptual truth. The difference in grounds between conceptual and intuitive truths motivates Bolzano's criticism of Kant's view that intuition plays the fundamental role in mathematics, a conceptual science by Bolzano's lights.

Moritz Schlick's work shaped logical empiricism and thereby an important part of philosophy in the first half of the twentieth century. A continuous thread that runs through his work is a philosophical diagnosis of the ‘great errors in philosophy’: philosophers assume that there is intuitive knowledge or knowledge by acquaintance. Yet, acquaintance is not knowledge, but an evaluative attitude. In this paper I will reconstruct Schlick's arguments for this conclusion in the light of his early practical philosophy and his reading of Schopenhauer's The World as Will and Idea. Having the historical roots of Schlick's dichotomy between acquaintance (intuition) and knowledge in view will put us in a position to explore and question its presuppositions.