An explanandum (L.) is a sentence describing a phenomenon that is to be explained, and the explanans is the sentences adduced as explanations of that phenomenon. For example, one person may pose an explanandum by asking 'why is there smoke?', and another may provide an explanans by responding 'because there is a fire'. In this example, 'smoke' is the explanandum, and 'fire' is the explanans.
As a siderbar, consider similar latinate terminology:
- Explicandum — that which gets explicated vs. Explicans — that which gives the explication
- Constitutum — that which gets made up, constituted vs. Constituens — that which makes it up, e.g. the constituents
- Definiendum — that which is being defined vs. Definiens — that which constitutes a definition
In accounting for the differences between artworks it is not enough to produce an explanation which merely restates difference, as this would simply echo the explanandum. e.g. It is no use saying that the difference between Gentile de Fabriano's Madonna and Child with Angels (1425) and Masaccio's Virgin and Child (1426) 'lies in' the stylistic differences between late Gothic and early Renaissance modes of figuration. That would merely present us with a restated difference to be explained (an explanandum), and not an explanation (explanans).
Nor, according to the Marxist art historian Frederick Antal does a difference in influence (regional, generational) upon the respective artists account for the essential differences: 'influences do not explain essentials'.
For Antal there must be a fundamental cause of the differences, and as an 'orthodox' Marxist art historian, this fundamental explanation can only be given through an understanding of the economic basis of class society: a fundamental social antagonism expresses itself in the differences between Gentile and Masaccio's stylistic approaches; the rational, natural, frugal, sober and realistic style of the latter expresses the values of a newly prosperous and briefly powerful bourgeoisie asserting itself, while Gentile's more courtly, geometricised and ritualistic style expresses a conservative outlook more befitting the aristocracy.
 Hatt, M. & Klonk, C., 2006, Art History: A Critical Introduction to its Methods, p. 132
 Antal, F., 1948, Florentine Painting and its Social Background, 9.3.