Full Idea
The dialectical principle, for Hegel, is the principle whereby apparently stable thoughts reveal their inherent instability by turning into their opposites and then into new, more complex thoughts (as being turns to nothing, and then becoming).
Gist of Idea
Dialectic is the instability of thoughts generating their opposite, and then new more complex thoughts
Source
report of Georg W.F.Hegel (Science of Logic [1816]) by Stephen Houlgate - An Introduction to Hegel 02 'The Method'
Book Reference
Houlgate,Stephen: 'An Introduction to Hegel' [Blackwell 2005], p.38
A Reaction
Houlgate says this is unique to Hegel, and is NOT the familiar thesis-antithesis-synthesis idea of dialectic, found in Kant and Engels. Hegelian idea shares the Greek idea of insights arising from oppositions.