First, Chisholm uses the terms ‘substrate’ and ‘content’ to indicate the object and property involved in an event. The substrate must be a contingently existing thing. The content must be a property that the substrate has contingently. Each event is said to have its substrate and content essentially. So if there was such an event as Plato’s writing of the Republic, then that event essentially involved Plato and the property of writing the Republic. The event itself could not have occurred with a different substrate or content. Nor could it have failed to have its “categorial” property of being an event.