bewildebeest,

step 1: thetical - Of the nature of or involving direct or positive statement; laid down or stated positively or absolutely; positive; dogmatic; arbitrary.
1653 H. More Conject. Cabbal. (1713) 66 This Law+was merely Thetical or Positive, not Indispensable and Natural. 1678 Cudworth Intell. Syst. Pref. 2. 1718 J. Chamberlayne Relig. Philos. Pref. (1730) 4 The Thetical Way+must not appear imperfect to them. 1873 W. Humphrey Div. Teacher p. iii, A thetical exposition of the Catholic doctrine. a thesis lays out something of this nature.

step 2: an antithesis is simply a proposal opposed to a thesis; so, antithetical is characterised by direct opposition.

step 3: arbitrary and dogmatic are sometimes thought to be pejorative in nature, but are not, perforce, negative.
arbitrary - 2. Law. Relating to, or dependent on, the discretion of an arbiter, arbitrator, or other legally-recognized authority; discretionary, not fixed.
dogmatic - 3. Proceeding upon a priori principles accepted as true, instead of being founded upon experience or induction