The best farm target appears to be Idolator Soul , since a guide snippet specifically says the relevant Harlowe class mod drops from that boss.

What that means

If you’re chasing the purple Relative Physicist mod for Harlowe, the safest bet is to repeatedly farm Idolator Soul rather than random enemies. The same source also points to specific boss farms for other Harlowe class mods, which suggests this mod is part of a dedicated drop pool rather than a general world drop.

Practical farm route

  • Go to the boss that drops Harlowe class mods from its dedicated pool.
  • Run the boss repeatedly until the mod drops.
  • If you’re not getting it, keep focusing on that dedicated target instead of spreading your farm across unrelated enemies.

Caveat

I only found one strong source snippet tying Harlowe class-mod farming to a specific boss, so this is the best verified lead rather than an absolute guarantee.

If you want, I can also help narrow down whether Relative Physicist is a dedicated drop, world drop, or tied to a specific boss phase.