Inventio, one of the five canons of rhetoric, is the method used for the discovery of arguments in Western rhetoric and comes from the Latin word, meaning "invention" or "discovery". Inventio is the central, indispensable canon of rhetoric, and traditionally means a systematic search for arguments.

Speakers use inventio when they begin the thought process of forming and developing an effective argument. Often, the invention phase can be seen as the first step in an attempt to generate ideas or create an argument that is convincing and compelling. The other four canons of classical rhetoric (namely dispositio, elocutio, memoria, and pronuntiatio) rely on their interrelationship with invention.

According to Crowley and Hawhee, invention is the division of rhetoric that investigates the possible means by which proofs can be discovered. It supplies the speaker and writers with sets of instructions or ideas that help them to find and compose arguments that are appropriate for a given rhetorical situation.

For personal and lyric essays, narratives, and descriptive writing, invention techniques help writers draw from their memory and observation for the kinds of details that will add depth to their essays.

The first direction of invention aims toward deriving heuristic procedures or systematic strategies that will aid students in discovering and generating ideas about which they might write; the second direction of invention is characterized by how writers establish "voice" in writing and realize individual selves in discourse.

One of the oldest criticisms of rhetoric is that as an art it has no proper subject matter. In other words, an orator might speak on any topic, with his success being measured purely on the brilliance of his rhetorical skills. This aspect of rhetoric is one reason why Plato attacked what he saw as empty rhetoric on the part of sophist philosophers such as Gorgias.

Aristotle, in his works on rhetoric, answered Plato's charges by arguing that reason and rhetoric are intertwined ("Rhetoric is the counterpart of Dialectic" is the first sentence of his Rhetoric). In Aristotle's view, dialectical reasoning is the mechanism for discovering universal truths; rhetoric is the method for clarifying and communicating these principles to others. And in order to communicate effectively, an orator must be able to assemble proper arguments that support a thesis.

Inventio, therefore, is the systematic discovery of rhetorical practices. In the Greek and Roman traditions, rhetorical practices are often but not always arguments. Aristotle, as well as later writers on rhetoric, such as Cicero and Quintilian, devoted considerable attention to developing and formalizing the discipline of rhetorical invention. Two important concepts within invention were topoi and stasis. Other rhetorical cultures seem to have additional means of locating "available means". Historian of Celtic poetics Robert Graves credited analepsis as a method of inventing his historical arguments in The White Goddess, and the Mazatec medicine woman Maria Sabina credited the hallucinogenic psilocybe mushroom with the flow of her discourse. The philosopher Jacques Derrida described inventio as the "invention of the other."