Personal, direct observation was what the Buddha taught; for example:
- "When you know for yourselves that, "These qualities are unskillful; these qualities are blameworthy; these qualities are criticized by the wise; these qualities, when adopted & carried out, lead to harm & to suffering" — then you should abandon them.'" AN 3.65
- "The Dhamma is well-expounded by the Blessed One, to be seen here & now, timeless, inviting verification, pertinent, to be realized by the wise for themselves." AN 3.70
As commonly claimed in other religions, the "word of a god" is revealed to special prophets in history which non-prophets must believe on blind faith. We have no way of directly knowing or verifying these utterances for ourselves, as these revelations are fixed in space and time, and given only to a special people group (e.g. prophets, priests, ethnic people group, hierarchies, etc.)
On the other hand, early Buddhism is based on universal, timeless truths which the Buddha himself observed and realized, the same truths which he calls his disciples to observe and realize for ourselves as well. Whatever the Awakened One observed, he calls us to observe and awaken to the very same things in the "here & now", the Laws of Reality which are "timeless" and invites "verification", to be "realized" and "known" by the "wise for themselves".
In his search and struggle for enlightenment and freedom from dukkha/suffering (the core problem which drives all the activities of humanity - the search for gods, saviors, food, sex, entertainment, etc.), the Buddha made Observations regarding various physical and mental states, developed testable predictions regarding various ascetic practices, and then subjected himself to those practices. He eliminated the practices which did not bring him to nibbana (total unbinding, freedom from dukkha), and presented his "General Theory" - the Dhamma - which he taught to his disciples. The same Dhamma which he realized, he invites us to test and realize for ourselves, to reach the same level of enlightenment and freedom which he himself did. In such a way, he practiced the scientific method.
(Note that many forms of late "Buddhisms" which developed centuries later have essentially morphed into religions which depend on blind-faith in various celestial-"savior" Buddhas or Bodhisattvas with established hierarchies, so I think an important distinguishing must be made between late and early Buddhism).