"Concepts can only acquire content when they are connected, however indirectly, with sensible experience. But no logic…"

"Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their ear…"

"A systematic revolution of basic concepts begins with Einsteinian science. In the very detail of its concepts a relat…"

"There are three aspects of thinking as more or less complete stages of it. These are, conception, judgment, and reaso…"

"Conception is taking together into one idea the element of meaning common to a number of objects and things. ...Conce…"

"Concepts have meaning only if we can point to objects to which they refer and to rules by which they are assigned to …"

"The belief in an external world independent of the percipient subject is the foundation of all science. But since our…"

"I believe that the first step in the setting of a "real external world" is the formation of the concept of bodily obj…"

"The aim of science is... a comprehension, as complete as possible, of the connection between the sense experiences in…"

"All concepts, even those which are closest to experience, are from the point of view of logic freely chosen conventions."