Bolzano has a complex theory of how we are able to sense things. He explains sensation by means of the term intuition, in German called "Anschauung". An intuition is a simple idea, it has only one object ("Einzelvorstellung"), but besides that, it is also unique (Bolzano needs this to explain sensation). Intuitions ("Anschauungen") are objective ideas, they belong to the "an sich" realm, which means that they don't have existence. As said, Bolzano's argumentation for intuitions is by an explanation of sensation.