You may be referring to the motto extracted from Spinoza: Omnis determinatio est negatioOmnis determinatio est negatio, every determination is negation. As applied to knowledge, it means that we know something by knowing what it is not, what it differs from. Spinoza's wording in his 1674 letter to a friend is not as succinct:
The one who transformed Spinoza's subordinate clause into a motto was none other than Hegel. In the Science of Logic he presented the "determinate negation" as the essence of his own dialectical method, and believed that Spinoza did not appreciate his own discovery: "Spinoza’s top-down determination starts with a single category (in his case, divine substance) that is then progressively divided by the application of concepts—the model being Plato’s method of division in which a genus concept is divided into particular species by the presence or absence of some differentiating property. From Hegel’s point of view, however, this cannot capture individuals as other than parts of that greater whole — a metaphysical picture in relation to Spinoza he refers to as acosmism".
"Spinoza’s top-down determination starts with a single category (in his case, divine substance) that is then progressively divided by the application of concepts—the model being Plato’s method of division in which a genus concept is divided into particular species by the presence or absence of some differentiating property. From Hegel’s point of view, however, this cannot capture individuals as other than parts of that greater whole — a metaphysical picture in relation to Spinoza he refers to as acosmism".
According to Hegel, notions proceed from abstract to (determinately) negative, to be resolved in the concrete, the negation of negation, their superior form. See more in Melamed's entry (ch. 10) in Spinoza and German Idealism volumeSpinoza and German Idealism volume.
An alternative is the French linguist Saussure's view that "in language there are only differences without positive terms... The entire mechanism of language, with which we shall be concerned later, is based on oppositions of this kind (e.g. between the word ‘father’ and ‘mother’) and on the phonic and conceptual differences that they imply".
"in language there are only differences without positive terms... The entire mechanism of language, with which we shall be concerned later, is based on oppositions of this kind (e.g. between the word ‘father’ and ‘mother’) and on the phonic and conceptual differences that they imply".
In other words, there is no intrinsic meaning to a concept as such, meaning is difference, concepts show themselves only in their differences from other concepts, "most precise characteristic is to be what the others are not". Derrida, an influential French continental philosopher, extended Saussure's view from language to philosophy, and came up with a neologism "différance", which fuses "difference" and "deferral". The word alludes to the indefinite deferral of "meaning beyond language", when meanings of words are invariably explained in terms of other words. "There is nothing outside the text", concludes Derrida, only the différance.