… I have chosen the structure of a relation. I have chosen certain conditions (rules that I have made up) that reveal themselves in the logic of the procedure.
This is from ‘Play it Again, Sam’  in Richard Serra: Writings Interviews (1994):
… The perception of the work in its state of suspended animation, arrested motion, does not give one calculable truths like geometry, but a sense of presence, an isolated time. The apparent potential for disorder, for movement endows the structure with a quality outside of its physical or relational definition.
… “We experience more than we can analyse” (A.N. Whitehead). “Sensibility is inclusive and precedes analytic awareness.” (Anonymous). In San Francisco they say, “Flash on it.”
There is no general rule as to which formal properties suffice to determine the structure of a relation. I have chosen the structure of a relation. I have chosen certain conditions (rules that I have made up) that reveal themselves in the logic of the procedure.
There is a difference between definite literal fixed relationships, i.e., joints, clips, gluing, welding, etc. and those which are provisional, non-fixed, “clastic.” The former seem unnecessary and irrelevant and tend to function as interposed elements.
This next is from ‘Shift’ in the same book:
… The boundaries of the work became the maximum distance two people could occupy and still keep each other in view.
… What I wanted was a dialectic between one’s perception of the place in totality and one’s relation to the field as walked. The result is a way of measuring oneself against the indeterminacy of the land. I’m not interested in looking at sculpture which is solely defined by its internal relationships. When you bounce a ball on a shifting ground, it doesn’t return to your hand.
… The work establishes a measure: one’s relation to it and to the land. One walks down the hill into the piece. As one does, the elements begin to rise in relation to one’s descending eye-level.
… The machinery of renaissance space depends on measurements remaining fixed and immutable. [By contrast] These steps [in his work ‘Shift’] relate to a continually shifting horizon, and as measurements, they are totally transitive: elevating, lowering, extending, foreshortening, contracting, compressing, and turning. The line as a visual element, per step, becomes a transitive verb.
Richard Serra, Shift, 1971-72