“The observer, when he seems to himself to be observing a stone, is really, if physics is to be believed, observing the effects of the stone upon himself.”

Lecture II : The Universal Categories, § 2 : Struggle, CP 5.53
Pragmatism and Pragmaticism (1903)

Ogni giusto re primo servatore dee essere delle leggi fatte da lui.
Seventh Day, Tenth Story
The Decameron (c. 1350)

The Need for Transcendence in the Postmodern World (1994)
Context: The relationship to the world that the modern science fostered and shaped now appears to have exhausted its potential. It is increasingly clear that, strangely, the relationship is missing something. It fails to connect with the most intrinsic nature of reality and with natural human experience. It is now more of a source of disintegration and doubt than a source of integration and meaning. It produces what amounts to a state of schizophrenia: Man as an observer is becoming completely alienated from himself as a being.

Source: Art As a Social System (2000), p. 54 as cited in: Pamela M. Lee (2004) Chronophobia: On Time in the Art of the 1960's. p. 66.

In der Quantenphysik dagegen bedeutet jede Beobachtung einen Eingriff in das Beobachtete; eine Zustandsveränderung am Beobachteten ist auf Grund der quantenphysikalischen Naturgesetze mit dem Beobachtungsprozess zwangslaüfig verknüpft. Also nicht ein sowieso, unabhängig von diesem Experiment vorhandener Tatbestand wird wahrgenommen, sondern wir selber rufen die Tatbestände hervor (oder: nötigen sie in bestimmter Richtung zu einer Klärung), die dann zur Wahrnehmung gelangen.
Quantenmechanische Bemerkungen zur Biologie und Psychologie, Erkenntnis, Vol. 4 (1934). p. 228.

Source: War and Peace

Source: The reality of the Mass Media (2000), p. 117.

The Rainbow of Desire (1995)
Context: Theatre has nothing to do with buildings or other physical constructions. Theatre — or theatricality — is the capacity, this human property which allows man to observe himself in action, in activity. The self-knowledge thus acquired allows him to be the subject (the one who observes) of another subject (the one who acts). It allows him to imagine variations of his action, to study alternatives. Man can see himself in the act of seeing, in the act of acting, in the act of feeling, the act of thinking. Feel himself feeling, think himself thinking.

Source: The Birth of Tragedy (1872), p. 15
Context: Thus the man who is responsive to artistic stimuli reacts to the reality of dreams as does the philosopher to the reality of existence; he observes closely, and he enjoys his observation: for it is out of these images that he interprets life, out of these processes that he trains himself for life. It is not only pleasant and agreeable images that he experiences with such universal understanding: the serious, the gloomy, the sad and the profound, the sudden restraints, the mockeries of chance, fearful expectations, in short the whole 'divine comedy' of life, the Inferno included, passes before him, not only as a shadow-play — for he too lives and suffers through these scenes — and yet also not without that fleeting sense of illusion; and perhaps many, like myself, can remember calling out to themselves in encouragement, amid the perils and terrors of the dream, and with success: 'It is a dream! I want to dream on!' Just as I have often been told of people who have been able to continue one and the same dream over three and more successive nights: facts which clearly show that our innermost being, our common foundation, experiences dreams with profound pleasure and joyful necessity.