Algorithmic composition progressed throughout the 20th century, as modernism became the dominant aesthetic, until finally ‘process music’ arrived, where the single remaining compositional decision related to what sonic resources to use. Composition pedagogy in the late 20th century did not explicitly include ‘expression’. This raises a second question, as repeatedly, musical discourse refers to ‘expression’, which seems to be something that audiences desire and to which they respond. Addressing these questions has lead me to reconsider the way music itself, and compositional processes, are characterised in music analysis. The outcome of my research is a new theory of music, using Fuzzy Logic principles. I am using this theory to build an algorithmic compositional decision-making system which can create specific aesthetic experiences.