Final answer:
The line in question is an assertion about the substance of dreams, hinting at their ephemeral nature but also their potential significance in revealing deeper truths.
Step-by-step explanation:
The line "Must dreams themselves be; seeing they're more slight" can be interpreted as an assertion about the substance of dreams. It suggests that dreams, despite their intangible and evanescent nature, have a form of existence. Dreams, being less substantial than our waking reality, pose questions about what is real and what is not. This links to the philosophical and psychological contemplations on the nature of dreams and their connection to our unconscious selves, as proposed by thinkers like Freud and Jung. Moreover, the poem from which the line is taken may explore the blurred boundaries between the dreaming state and waking life, suggesting that the content of dreams, while fleeting, may have significance or insight into deeper truths.