First gather all the evidence, then surveyed the total evidence to arrive at the most probable hypothesis. Deductions were made from the hypothesis; then the theory was further tested against new evidence, revises if need be, until finally the truth emerged with a probability close to certainty.
3 qualities necessary for the ideal detective: power of observation, deduction and knowledge.
It is useful to know something about human nature and what motivates people
Ask: What is in their Interest to do? You can never foretell what any one man will do, but you can say with precision what an average number will be up to. Individuals vary, but % remain constant
Don't forget common sense
I make it a rule, in all cases, to proceed on the strictly classical lines of Inductive Inquiry - Collect Facts, Make Hypotheses, Test them and seek for Verification. And I always endeavour to keep a perfectly open mind.
To obeserve correctly and decide wrongly is sure to happen to the best of us, but to observe carelessly happens only when we permit it.
See things for what they are.
"The greatest sign of an ill-regulated mind is to believe things because you wish them to be so." (Louis Pasteur)
Never jump to conclusions and try to collect facts as open-minded as possible.
We approached the case...with an absolutely blank mind...We had formed no theories. We were there simply to observe and to draw inferences from our observations.
It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
Observation - Start with collecting facts and follow them where they lead
Don't be too Quick. Let us know a little more before we act.
We can't observe or collect facts without some kind of view - what to look for, how to look and how to interpret what we see
What are the Facts? Holmes first gathered enough evidence - both positive and negative - that was relevant to his problem
Make sure Facts are Facts. I realize that if you ask people to account for "Facts", they usually spend more time finding reasons for them than finding out whether they are true.
Separate the relevant and important facts from the unimportant or accidental. It is of the highest importance in the art of detection to be able to recognize out of a number of facts which are incidental and which are vital.
There may be many theories that fit the facts.
Facts don't lie but we may have interpreted or stated them wrong and therefore drawn the wrong conclusion
More information isn't necessarily better information but it may falsely increase our confidence - What is not worth knowing is not worth knowing
Know where to look. We all see, but often you do not observe. It was invisible, buried in the mud. I only saw it because I was looking for it.
The eye sees only what it is trained to see. I see no more than you, but I have trained myself to notice what I see.
We need to both observe the big picture - forest and the details - trees.
What we see is all we think is there - What often leads us astray in an investigation is that we adopt the theory which is most likely to account for the "visible" and found facts but what if the important is left out? What is not reported, withheld, hidden?
Use the simplest means first
Sometimes things are not as simple as they seem. But sometimes they are not as complex as they seem, either.
What normally happens in similar situations? Why should this be any different
History often repears itself
What does this case resemble? What is the same between this situation and others?
Sometimes we learn more by looking for differences - not similarities - in situations
Patterns repeat but not always - Is it the same situation or circumstances or is it unique and fundamentally different from the past - what factors, conditions, behavior differ?
Remember that we see what we are looking for - if we look for similarities, this is what we see, if we look for the differences, that is what we find
The absence of something we expect to see or happen is information and a clue in itself.
Small pieces of information may in themselves look to be of no importance but may clarify things when taken together.
Eliminate possibilities - What can we exclude? By the method of exclusion, I had arrived at this result, for no other hypothesis would meet the facts.
Test our theory - if it disagrees with the facts it is wrong.
Check for other possible explanations - What else can explain this? One should always look for a possible alternative and provide against it. It is the first rule of criminal investigation. Have you any alternative theory which will meet the facts?
Patience - Take time to think things over. And when we don't get anywhere - accept doing nothing and wait until more evidence is available
Distance gives perspective - Sometimes we need to remove ourselves from the proble and get a fresh perspective
If we could see the world the way others see it, we easier understand why they do what they do
Don't make the world fit your tools and use the right tool for the job
A rule is only a rule if it's always true.
Update our beliefs in light of new information.
Criticize ourselves - Have we tried to find evidence against what we believe? Why might we be wrong? What have we overlooked? What new information or evidence is needed to make us change our mind? When we meet a fact which contradicts a prevailing theory, we must accept the fact and abandon the theory, even when the theory is supported by great names and generally accepted.
When we get better understanding or the facts or evidence don't agree with the theory we must change the theory and change course.
Learn from your Mistakes - and learn the general lessons. We should all have the desire to reduce our errors to the minimum and to eliminate entirely those due to careless observations and slovenly habits of thinking. To observe accurately, to reason clearly, to hold ourselves to as high a standard of efficiency as our equipment permits, are within the powers of all.
Deal with things that really matter and that we can do something about
Don't think about how to get things done, instead ask whether they're worth doing in the first place
A lot of misery comes from what we allow ourselves to get dragged into
We shouldn't disregard even a small probability. We have no right to take anything for granted.
The Future is hard to predict