It is virtually impossible to ask the question ‘what exactly is an idea’ without thinking, immediately, of Plato. In his Dialogues Plato used ‘idea’ to mean at least three things. We can talk of ‘the idea of beauty’: all things beautiful must have something in common that makes them beautiful, they all share the idea of beauty. Therefore, in every beautiful thing the idea of beauty is present, and it is what makes it beautiful. Here ‘idea’ refers to a universal quality or form in the mind. Secondly, the idea can, in Plato’s work, also be a paradigm: then ‘the idea of beauty’ refers to something which itself is more beautiful than anything else. It then stands for the perfect, against which all the other beautiful things are set off. Thirdly, Plato’s ‘idea’ can also be a purpose, or an end, to approach as perfectly as possible. The main point is that to Plato ‘the world of ideas’ is a separate reality, and a more fundamental reality than ‘the world of phenomena’. All visible things or known moral rules are reflections of the world of ideas. The fleeting thing mirrors the eternal idea. We have the individual human, who is a mortal, and we have the idea ‘human’, which is eternal. Or, we have the individual act of generosity (which is fleeting) and we have the idea ‘good’ (which is eternal).
Aristotle’s understanding of the relationship between ideas and phenomena was very different. To him only the individual human being (or the individual act of generosity) really existed. ‘Man’ or ‘humanity’ as a general idea is no more than an abstraction, or a name, which only exists in our thoughts; ideas for Aristotle therefore do not constitute a reality per se.
In Medieval philosophy this dichotomy (Plato versus Aristotle) lived on in the extended debates between realism (the reales, followers of Plato who claimed that the idea is an objective reality) and nominalism (claiming, like Aristotle, that the idea is but a name or nomen). The implication of realism is that general categories and values exist as a reality outside of human consciousness. In nominalism they are confined to consciousness. During the Middle Ages philosophy was dominated by the reales.
From Descartes onward, however, the ‘idea’ is generally seen as a mental content, a mind’s construction, existing in the mind. The idea became the human representation of the objective reality of phenomena. While Descartes thought that fundamental ideas like ‘the good’ and ‘the true’ are inherited, Locke reconnected ideas to empirical observation. Ever since Kant the human subject as a creator or constructor of ideas and thoughts has taken centre stage: our knowledge does not give us access to the Ding an sich, but concerns the things as they have been structured by a priori ideas in our reasoning. In spite of its strong influence, this Kantian idealism has been contradicted by several philosophers, especially in the first decades of the twentieth century, who caused a revival of – new forms of – realism.
I believe the historian of ideas should be well aware of this very old philosophical debate on the status of ideas (and of thought and human knowledge). However, this doesn’t mean that the historian could not find reasons to introduce his own definition of ideas. Ideas as a force in history deserve to be studied, whether they are seen as a distinct reality in their own right, or as constructions of and in the mind. History itself is an idea (mental construction) as well as an objectification of time (or duration). None of Plato’s definitions of the idea (the universalist, the paradigmatic, nor the teleological) seems to apply to the idea of history. Maybe history has to be seen as a sui generis.