In my head there is a permanent opposition-party; and whenever I take any step or come to any decision — though I may have given the matter mature consideration — it afterward attacks what I have done, without, however, being each time necessarily in the right. This is, I suppose, only a form of rectification on the part of the spirit of scrutiny; but it often reproaches me when I do not deserve it.

[In meinem Kopfe giebt es eine stehende Oppositionspartei, die gegen Alles, was ich, wenn auch mit reiflicher Überlegung, gethan, oder beschlossen habe, nachträglich polemisirt, ohne jedoch darum jedesmal Recht zu haben. Sie ist wohl nur eine Form des berichtigenden Prüfungsgeistes, macht mir aber oft unverdiente Vorwürfe.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 2, ch. 26 “Psychological Observations [Psychologische Bemerkungen],” § 345 (1851) [tr. Saunders (1890)]
    (Source)

(Source (German)). Alternate translation:

There is in my mind a standing opposition party which subsequently attacks everything I have done or decided, even after mature consideration, yet without its always being right on that account. It is, I suppose, only a form of the corrective spirit of investigation; but it often casts an unmerited slur on me.
[tr. Payne (1974)]