Difference between reasoning based on facts and reasoning based on assumptions

I want to make this as simple as possible, so I will start by explaining reasoning first.

Reasoning usually comes in form of premises and conclusion.

Deductive reasoning is more logically accurate, but it can be based on assumptions.

Example:

Premise 1:

If it rains today, I will bring an umbrella.

Premise 2:

It rains today.

Conclusion:

I will bring an umbrella.

We see that as long as premise 1 and premise 2 are true, conclusion logically follows.

So is this reasoning based on facts? 

No, not even close.

This is because none of the premises are proven to be true.

Premise 2 is an assumption.

There is no proof it rains today.

Premise 1 is an assumption too.

There is no proof that I will bring an umbrella if it rains.

It is not disproven that it rains today and I wont bring an umbrella because I forgot and was in a rush, or maybe I just like walking in rain without an umbrella, or it didnt rain much and I figured I wouldnt need it.

So since every premise is essentially unproven assumption, this reasoning is based on assumptions.

The other form of reasoning, without the use of "if" would be inclusion, category reasoning.

Premise 1:

I will bring an umbrella on days when it rains.

Premise 2:

It rains today.

Conclusion:

I will bring an umbrella today.

Despite changing from "if" to inclusion, it still stands that every premise is an assumption.

The claim "I will bring an umbrella" is included in category "on days when it rains".

But this inclusion is an assumption.

It could be possible that there are days when it rains and I dont bring an umbrella, therefore making the premise an assumption.

Since its not proven that I will bring an umbrella on days when it rains, the premise is essentially unproven.

Another form of reasoning is options reasoning, which is "this or that".

Example:

Premise 1:

It wont rain

or

I will bring an umbrella.

Premise 2:

It rains.

Conclusion:

I will bring an umbrella.

Since "or" in this reasoning means that when one option is true, the other is false,

when "it wont rain" is false, then "I will bring an umbrella" is true.

But premise 1 is still an assumption, because both options can be true.

It can be true both that "it wont rain" and that "I will bring an umbrella", since people can obviously bring an umbrella even on days it doesnt rain.

Also, they can both be false.

It can be false that it "it wont rain" and it can at the same time be false that "I will bring an umbrella".

So what is reasoning based on facts?

Its reasoning where every premise is a fact.

Example:

Premise 1: 

Sun exists.

or

It is false that Sun exists.

Premise 2:

Sun exists

Conclusion:

The claim "It is false that Sun exists" is not true.

Premise 1 is a fact, since Sun can logically either exist either not exist, but not both and not neither.

Premise 2 is a fact, since Sun is observable to exist.

Conclusion then logically follows.

So, reasoning based on facts is reasoning where every premise is an observable fact, and conclusion logically follows from true premises.

This reasoning uses "if" premise when that premise can be observed.

For example, when we observe that "if there is source of light, there will be light".

It uses inclusion when inclusion can be observed.

Example:

"Not all dogs have tail"

It uses "or" when logically, options are presented which cant all be true,

where one of them must be true, where all except one are false,

where there are no other options, such as Sun which either exists  either doesnt.

That is reasoning where every premise is an observable fact.

I hope you are planning to go to law school.

That is a truism. Anyone can create a truism, but it is how you utilize it in your argument that is crucial.

Yes.

But many choose reasoning based on assumptions instead.

I am saying that reasoning should have facts as premises, not assumptions as premises.

While anyone can create truisms or search for facts, many people cant tell the difference between facts and assumptions.

In fact, many choose to support assumptions with assumptions, which doesnt get them any closer to reasoning based on facts.

Creating a logical system based on assumptions is somewhat usefull, because you can explore the implications of you assumptions. Then we can check those implications up against facts. For example, assuming Biblical inerancy, a global flood is implied. We can then check in with science to find out if a global flood did occur. Since science shows us a world in which that never happened, the base assumption must be wrong. Hence, Biblical erancy is proven. 

The problem is when people never check the implications of their assumptions up against facts, or when they selectively ignore facts that clash with their conclusions. 

You don't always have to start out with facts, but you have to bring them in sooner or later. 

Thats called building facts from assumptions.

"If assumption A is true, then B follows."

This claim can be a fact even if assumption A isnt a fact, even if assumption A is wrong.

B doesnt follow, therefore, assumption A is wrong.

You mentioned global flood.

"Global flood happened" is an assumption.

"If global flood happened, there would be traces of it" is a claim which is a fact.

So facts can be built from assumptions, but they must still be facts as a whole.

Now, regarding the global flood from Christian view, you have to understand that you are dealing with people who believe in magic.

In their view, God, for some reason, removed all traces of flood after flood happened, and God also created traces of evolution despite that, in their view, evolution didnt happen.

So one is only left with a question why would God do that.

People also have a tendency to exaggerate and write associated folk tales.

And some people like to think that such folk tales are true and accurate.

Whereas the truth was that there was a local flood event and a woman named Norah rescued some goats in a rowing boat.

This was an assumption, but more logical and less exaggerated.

Certain Logical propositions are axiomatically true, because of their structure. The fallacies occur when the content is wrong or something else except true. As an example,

p -> q

p

therefore, q.

Now returning to the deduction, the conclusion of the deduction is always true given the premises are true. But if the premises are based on the induction or even an assumption, then the truth of conclusion is dependent on the truth of the premise. Best option would be to apply probabilistic reasoning. Sometimes that can be unrealistic, unnecessary or even stupid.