The Reason

The Reason Third Case behaves differently from the other two usages, since it does not directly link to a verb but rather marks a reason clause in a sentence. Some would say it is a non-case usage included in the Reason Indicators (རྒྱུ་མཚན་སྟོན་པའི་སྒྲ།).

It's paradigm form is “by reason of...” (-འི་རྒྱུ་མཚན་གྱིས།) which is usually abbreviated to just -ས།.

For example:

• “The darkness was dispelled because the sun rose.” ཉི་མ་ཤར་བས་མུན་པ་བསལ།
  i.e. ཉི་མ་ཤར་བའི་རྒྱུ་མཚན་གྱིས་་་
• “It cannot be seen with the eyes due to being far away.” རྒྱང་ཐག་རིང་བས་མིག་གིས་མཐོང་མི་ཐུབ།
  i.e. རྒྱང་ཐག་རིང་བའི་རྒྱུ་མཚན་གྱིས་་་

This first example illustrates why the Third Case marks reasons as well as agents, since another way of reading it could be “The darkness was dispelled by the rising of the sun.”

This arises due to the close relationship between reasons and causes:

• The word “reason” in this context (རྒྱུ་མཚན།) literally means “signifier of the cause” (in English “because”)
• The sun rising causes the darkness to be dispelled in a similar way to how Tashi and the pen (as agent and instrument) cause a letter to be written.
• Therefore, a reason is seen as a causative / agentive activity or state of being, whereas an agent is seen as a causative noun.

As such, a good rule of thumb is:

• When marking a noun it will be a primary agent, secondary agent or instrument.
• When marking nominalised verbs, auxiliaries, particles, or adjectives it will be a reason.

See: Reason Indicators
Up a level: The Third Case – Agentive