Hermann Grassmann's ideas on the nature and foundations of mathematics were published as an integral part of his mathematical treatise, the Ausdehnungslehre, in 1844. In spite of its notoriously obscure style we can better understand the work if we view it as an expression of the dialectical philosophy of his mentor, the theologian F. Schleiermacher. The relation to Schleiermacher is presented here through an analysis of the principal ideas of the Ausdehnungslehre.