 Some of their most important contributions are accounts of the distinction between presentations (perceptions and imaginations) and the acts of meaning something with an expression; of affective phenomena, including episodic emotions and long-lasting sentiments; of knowledge as a direct grasp of objects and states of affairs rather than any sort of belief or conviction; of the distinction between attitudes or stances, such as belief and emotions, on the one hand, and knowledge, on the other; of the distinction between motivation and reasons, on the one hand, and mental causality, on the other; of the structure of speech acts such as promises, orders and questions; of collective intentionality and its relation to social facts; and of values, our acquaintance with them, and our reactions to them. In ontology, they developed theories of states of affairs and an account of modality according to which the most fundamental type of necessity flows from the essences or natures of objects. The fact that their ideas have been consistently overlooked is made even more curious by the fact that they write in clear German and that many of their ideas were rediscovered, beyond phenomenology, during the twentieth century.