The speaker celebrates that another poet has been using the beloved as their muse, but insists that this rival poet's art is merely decoration, while the speaker's is genuine.
Sonnet 78 begins with gracious acknowledgment ('So oft have I invoked thee') and ends with bitter defensiveness ('Yet be most proud of that which I compile'). The speaker admits that the beloved's beauty has become a common muse ('every alien pen hath got my use'), but claims this is no threat because the rivals merely 'mend the style'—they polish surface without substance. This is almost convincing until lines 13-14 reveal the real anxiety: the speaker needs the beloved to elevate his 'rude ignorance' to 'learning.' The beloved is not merely subject but transformer. If the beloved is equally transforming other poets, the speaker's special claim dissolves. The defensive assertion that he is 'all my art' reads as anxious overcompensation.
The sonnet reveals an untenable position: the beloved's beauty is inexhaustible and worth celebrating, yet this very inexhaustibility threatens the speaker's monopoly. If the beloved can inspire 'every alien pen,' then their inspiration is not singular or special. The speaker tries to maintain distinction by claiming qualitative superiority (substance vs. decoration), but the logic is circular: the beloved makes other poets better, but makes the speaker special. This works only if the beloved's influence on the speaker is transformative in a way it is not for others. Yet the evidence (lines 5-8) suggests universal elevation. The sonnet thus stages an impossible argument: the speaker wants to be the only one who truly benefits from the beloved's presence while watching that presence multiply.
A musician whose muse has been discovered by a trendier competitor says: 'Yeah, other artists are making songs about them now. But look, they're just doing pretty covers. My music is where the real soul is.' The defensiveness shows through the bravado. The beloved is no longer solely the speaker's property to immortalize.