 The second use of the concept is more specific. According to this, substances are a particular kind of basic entity, and some philosophical theories acknowledge them and others do not. On this use, Hume’s impressions and ideas are not substances, even though they are the building blocks of—what constitutes ‘being’ for—his world. According to this usage, it is a live issue whether the fundamental entities are substances or something else, such as events, or properties located at space-times. This conception of substance derives from the intuitive notion of individual thing or object, which contrast mainly with properties and events. The issue is how we are to understand the notion of an object, and whether, in the light of the correct understanding, it remains a basic notion, or one that must be characterised in more fundamental terms. Whether, for example, an object can be thought of as nothing more than a bundle of properties, or a series of events.