The logic system of the ancient Indians was very advanced. There were many ways of deriving rational knowledge.
When you have direct substantial eviedence on hand with no ambiguity, it is known as Hasthamalaka Nyayam. Hastha aamalakam is uLLangai nellikkani. But there are other ways of deducing without a direct source of knowledge. One of them is the Arthabathi nyayam.
Time is already 12-05 am. I am not supposed to keep awake so late. Thats what all my friends in the Web say. I must obey them. Tomorrow I will explain what it is and how it is applied and what you can do with it in your researches.