HERE is the BBC Horizon programme No Ordinary Genius on Richard Feynman.
From the video at 5:13 here is Feynman on guesses and observations in science.
Richard Feynman lecturing at the chalkboard
Now I am going to discuss how we would look for a new law.
In general we look for a new law by the following process.
First we guess it.
(Laughter from audience)
Don't laugh that's extremely true.
Then we compute the consequences of the guess.
To see what, if this law is right, to see what it would imply.
And then we compare the computation results to nature or we say compare to experiment or experience.
Compare it directly with observation to see if it works.
If it disagrees with experiment, it's wrong.
In that simple statement is the key to science.