I went through one (1) semester at the Ateneo de Davao University
College of Law trying to understand the concept of obligations.  

An obligation is defined under the New Civil Code. 
According to the law, "An obligation is a juridical necessity to GIVE, to DO
and NOT to DO.

Obligations arise from  five sources of obligations, namely:

1. Law;
2. Contracts;
3. Quasi-contracts;
4. Acts or omissions punished by law; and
5. Quasi-delicts.

I wish to include:
 
6. Pagtas.

In my mind, although technically speaking, pagtas are not
enforceable in civil courts, these are essentially,
sources of natural obligations.