skey-
“to cut, to split”cut, divide
Root for cutting, yielding Latin scindere, Greek skhizein (to split), English shed/sheath.
Discussion
The PIE root *skey- (to cut, to split, to separate) produced one of the most cognitively significant word families in the IE vocabulary — a set of descendants that connects physical cutting to intellectual analysis, as if the mind's work of understanding were conceived as a form of blade-work.
Latin scīre (to know — originally "to cut, to separate, to distinguish") shows the most dramatic semantic development: from cutting to knowing. The path runs through "to separate" → "to distinguish" → "to discern" → "to know." From scīre English inherits: science (scientia, "the state of knowing"), conscience (con-scientia, "knowing together with oneself" — inner awareness), conscious, omniscient (all-knowing), prescient (knowing beforehand), and nescient (not knowing). The entire Western vocabulary of scientific knowledge rests on a PIE cutting metaphor.
The physical cutting sense survives in Latin scindere (to cut, to tear apart) and its English derivatives: scissors (via Medieval Latin), schism (through Greek skhísma, "a split" — from the same root), rescind (to cut back, to annul), and abscission (cutting away).
Greek skhízein (σχίζειν, "to split, to cleave") preserves the cutting sense and gave: schizophrenia ("split mind"), schist (a rock that splits in layers), and the prefix schizo- in clinical vocabulary.
Old English scēadan (to separate, to divide — ancestor of English shed, "to shed light/tears/leaves" = to separate and release) and the related scield (shield — a split piece of wood used for protection) continue the Germanic reflexes.
The root's conceptual arc — cutting → separating → distinguishing → knowing — is one of the most revealing semantic developments in the IE vocabulary. It encodes a theory of knowledge: to know something is to have cut it apart mentally, to have separated the relevant from the irrelevant, the true from the false. Science is, etymologically, the art of cutting.
Notes
Pokorny 920-922. English science, schism from this root.