A rational bond (الجامع العقلي) is a relationship the intellect recognizes through shared identity, relevant similarity, or intelligible correlation. Identity/unity (اتحاد) may involve speaking of one subject through several acts. Similarity (تماثل) relates different subjects with a relevant common attribute.
Example 1: “This person teaches and researches” brings two related actions under one subject.
Example 2: “Zayd is a scholar, and ʿAmr is a jurist” relates distinct people through intellectual work.
Similarity never requires two entities to match in every respect. The jāmiʿ is the particular shared aspect relevant to the speaker’s purpose.