This chapter probes the conceptual architecture of irritability in the eighteenth century. It justifies this case study not through a pre-established research agenda but because automated statistical comparisons reveal a marked transformation both in the term itself and in the broader network in which it is embedded. Irritability has long been marginalised in favour of its sister term, sensibility; yet we demonstrate the abiding significance of the former, in a variety of canonical works (Erasmus Darwin, Edmund Burke) and less familiar medical handbooks. This largely overlooked medical discourse infuses broader thinking on gender, colonialism and aesthetics; it worries the distinction between human and non-human life. We conclude by proving that the emergence of the irritability network holds significant consequences for other forms of conceptual thinking. In particular, we show how it affords a rethinking of the notion of habit, and facilitates the transformation of the cultural concept of system from a largely Newtonian and mechanistic notion, at the beginning of the eighteenth century, to an increasingly dynamical and physiological entity.