"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense."
-- Attributed to the Buddha (5th century B.C.E.)
"Nullius in verba."
"On no man's word."
-- Motto of the Royal Society since 1663
Investigation is about finding things out. But why would you want to find things out in the first place? What are the reasons behind your efforts?
The reasons vary. Academics often explore a particular topic with the goal to increase knowledge about that particular topic, knowledge for knowledge's sake. Government researchers study issues in support of legislative action and public policy. Police officers conduct investigations as part of their law enforcement duties. A hobbyist may delve into a topic out of curiosity.
Often the purpose of investigation is to provide information for a decision that must be made.