Blake compares an unchanging opinion to stagnant water, suggesting that thought becomes unhealthy when it is never examined or revised. The image of “reptiles of the mind” gives intellectual rigidity a corrupting quality: ideas that remain motionless can foster error, prejudice or distorted thinking.

The quotation does not imply that opinions should change without reason. Rather, it presents willingness to reconsider a belief as part of active thought. A person who encounters new evidence, experience or argument should remain capable of adjusting a previous judgement instead of treating consistency as an end in itself.

Within The Marriage of Heaven and Hell, the statement fits Blake’s wider emphasis on movement, opposition and transformation. Its central concern is intellectual flexibility: thought remains productive when beliefs can be questioned, tested and altered rather than allowed to become fixed through habit or certainty.