Allow me to give you some emotional and practical reinforcement:

I want you to envision that you are holding a steel hammer in your right hand, with your left hand resting on a table in front of you, palm down. Now imagine violently crushing the bones in your hand with said hammer...

How confident or un-confident are you that you can actually break your finger bones with a hammer?

How confident are you that having broken finger bones is a reliably painful or undesirable thing?

Do you feel the need to question the subjective value you retain by not harming yourself in such a way?

You may doubt the external "realness" of pain and pleasure, but you cannot deny the intrinsic value of pursuing one and avoiding the other.

The trick in philosophy with regards to an absence of absolute certainty is that we must instead compare premises and conclusions on a spectrum of weak to strong (some arguments and conclusions are much stronger and more reliable than others). If you break three fingers and a thumb on your left hand, how compelled would you be to smash the remaining finger for the sake of increasing certainty?

But how can conclusions be stronger or weaker when we cannot know the strengths or weaknesses of the arguments?

Strengths and weaknesses would also be uncertain?

This causes me anxiety. — Kranky

You seem to be confident enough that you feel anxious. So... it would appear that some things are not in doubt after all.

How can I live my life (if I even exist) doubting that every thought that enters my mind as being real and true. — Kranky

This is a game you are playing with yourself. You can go round and round with this nonsense and spin yourself into a tizzy.

So how is it best to stop?

But how can conclusions be stronger or weaker when we cannot know the strengths or weaknesses of the arguments?

Strengths and weaknesses would also be uncertain? — Kranky

We can test the strength of arguments through experimentation and empirical/observable evidence.

If I say that smashing your last finger with a hammer won't cause you harm, what could you use to cast doubt on the strength/truthiness of that claim?

Repeatable experiences reveal consistent relationships, and they're consistent enough that we can say the sun will rise (even though it might not) without a reasonable fret.

Forget about absolute certainty and perfect truth -- whatever that is. What we can obtain is a reasonable amount of uncertainty. Two concepts: "reasonable" and "uncertainty". Stick with what is reasonable, manageable and obtainable. Perfect certainty is unreasonable and neither obtainable nor manageable. We can live with an acceptable degree of uncertainty. How much is that? Well, that's for individuals to decide. If you can't stand uncertainty, then don't invest in the stock market. Don't gamble. Get vaccinated. Some people like the sharp taste of risk. For them the stock market is an elixir.

So how is it best to stop? — Kranky

Think about something else! Read a good book. Clean up the kitchen. Try to fix the broken alarm clock. Binge watch a series. Rake up the leaves. Go for a run. Do some yoga. Really, almost anything. There are a million things more productive than worrying about not being certain of anything.

You have become an extreme skeptic. This is unfortunate since Skepticism is a flawed philosophy.

There are actually many, many things you may be quite certain of, however you are probably too afraid to experiment scientifically and find out.

For example, if you step in front of a semi truck moving at 60 miles per hour you will most certainly be struck and killed. So what is it about the next life that you are so afraid of such that you are too afraid to put your skepticism to the acid test ?!

On a less sinister note, you can perform any action and be quite certain that you were the performer of that act. Descartes uses thinking for example. Cogito ergo sum. I think (an action) therefore I am (exist).

Descartes nailed the final nail (his action) into the coffin of extremist Skepticism.

You should review and refresh your readings (an act) about Descartes.

Any body part besides and including the hand would work in this kind of experiment.

This is yet another proof that pain ultimately teaches us that we are actually alive.

Bravo !!!

@Bitter-Crank you have nicely articulated a corollary to Descartes.

You are the winner.

Aristotle would be proud. Ok now I want to know who is your own favorite philosopher?

… Remembering also of course that Philosophy involves thought experiments whereas Science involves physical experiments.

So far @Bitter-Crank is the winner of this debate on purely philosophical grounds.

Suppose everything is uncertain, why would it cause you anxiety? Anxiety would mean you have interpreted something as certain within all the uncertainty that your life is. Why?

Reason suggests that, if everything is uncertain then what you've been learning about and interacting with throughout your life so far is that very uncertainty. So the only certainty you have is that you know this uncertainty. And if you know it, why be anxious?

So far Bitter-Crank is the winner of this debate on purely philosophical grounds. — hks

Thank you, but winning a debate on philosophical grounds and 50¢ won't get me a cup of coffee.

I have come to the conclusion that we can be certain of nothing. — Kranky

What you experience is real to you, and that's what matters. Logic cannot tell you what is going to happen next with absolute certainty, all it does is relate some of your experiences with some others. But why is it that you long for absolute certainty? You say this causes you anxiety, and indeed do you not long for absolute certainty precisely because you fear death? Because you do not feel in control of your life, because you need to regain control, and logic and reason have helped you in the past but you see that they do not help you all the way.

Absolute certainty is the absence of change. But you can see change as the absolute root of existence, since the absence of change is the absence of existence. So in looking for absolute certainty you are looking for death. But you don't want death. So stop looking for absolute certainty.

Deep down it's not the lack of absolute certainty that makes you anxious, there is something else that makes you anxious, and you looked for a solution in absolute certainty, but you haven't found it, so you blame it on the lack of absolute certainty, but the root cause is something else.

I've come to believe that the real me has decided to experience life as a human being. Having accepted that I reason that my mortal existence is real but not ultimately real, so the trick is to hold those truths and carry on with the game, after all there must be a good reason for this drama production.

I have come to the conclusion that we can be certain of nothing.

It appears that I'm posting on this forum.
I may not be and cannot be certain that I am. — Kranky

There are a few things you can be certain of.

1. You can be certain that it appears to you that you are posting here; and since it could not appear to you that you are if you did not exist, because there would be no-one for it to appear to, you can be certain that you exist.

2. Since it appears to you that you live in a world that is stable and obeys fixed rules (e.g. the law of gravity, the fact that objects stay in their places and don't suddenly vanish or change into other objects), you can be certain that there is something real that is causing that appearance of stability.

3. You can be certain that whatever is real and is causing the appearance of stability is not you, because if it was you, you would know (this is why solipsism is false). So you can be certain that there is something real and external to you that causes your world to appear stable.

4. You cannot be certain what this external real thing is: a realist would say it was the objects around you, Kant would say it was noumena, Berkeley would say it was God. This is all interesting speculation, but it doesn't actually matter to you which of them is right, because you do not live among these underlying real objects, you live among the appearances. Since the appearances are always stable, it follows that whatever is real and is causing them must also be stable; so you can be certain that there is something real and stable underlying the appearances, even though you can't be certain what it is.

One more thing: if philosophy makes you unhappy, give it up and find something that makes you happy. A lot of philosophy gets done, but hardly any of it is of any importance to anyone but other philosophers; mostly it's just a game, the thinking person's Sudoku. If it makes you depressed or worried, give it up and find something that doesn't.

This causes me anxiety. How can I live my life (if I even exist) doubting that every thought that enters my mind as being real and true. — Kranky

You can assume other people are real which helps.

Starting with 'I think therefore I am', if you treat self as just the conscious train of thought of your mind, you can say 'You think, therefore you are' as when you are in a conversation, it is clear that the other 'voice' is a separate train of thought and thus a separate individual by the definition I used. So on this basis I think you can dismiss solipsism.

This causes me anxiety. How can I live my life (if I even exist) doubting that every thought that enters my mind as being real and true. — Kranky

During our dreams we do not know we are dreaming. We may even dream of interpreting a dream. Only on waking do we know it was a dream. Only after the great awakening will we realize that this is the great dream. And yet fools think they are awake, presuming to know that they are rulers or herdsmen. How dense! You and Confucius are both dreaming, and I who say you are a dream am also a dream. Such is my tale. It will probably be called preposterous, but after ten thousand generations there may be a great sage who will be able to explain it, a trivial interval equivalent to the passage from morning to night.
________________________________________
To the most trivial actions, attach the devotion and mindfulness of a hundred monks. To matters of life and death, attach a sense of humor. — Chuang Tzu

One cannot, it seems, awaken oneself by indulging in anxiety, because that too is part of the dream. There is nothing for it but to take the dream as real in the meantime; feed the seeming body, care for the seeming friends, and worry about real reality when one awakens to it.

Row, row, row your boat
Gently down the stream,
Merrily merrily, merrily, merrily
Life is but a dream. — traditional

Thanks for replys.

My question:

If a car was heading towards me at great speed, I would, perhaps, conclude that this would kill me

This conclusion could be built upon the premis of me believing fast cars kill if hitting a pedestrian.

However even my reasoning for this (fast car = death) would not be certain. I could, in theory, be standing in front of the car believing there is no danger and a clever demon making it seem otherwise.

Is this possible?

Hopefully, something smarter than the speculator in you will take over command at that moment.

Or if your prefer the thought from a dead French guy:

"Philosophy triumphs easily over past evils and future evils; but present evils triumph over it.”
― François de La Rochefoucauld, Reflections; or Sentences and Moral Maxims

However even my reasoning for this (fast car = death) would not be certain. I could, in theory, be standing in front of the car believing there is no danger and a clever demon making it seem otherwise. — Kranky

You think 'a fast car could kill if it hits you' is less real than a demon?
You should be afraid. Be very afraid.

I would buy you a drink and/or coffee if I could. We all owe you a drink at least for your purely philosophical refutation of @Kranky's extremist skepticism.

Thanks for replys.

My question:

If a car was heading towards me at great speed, I would, perhaps, conclude that this would kill me

This conclusion could be built upon the premis of me believing fast cars kill if hitting a pedestrian.

However even my reasoning for this (fast car = death) would not be certain. I could, in theory, be standing in front of the car believing there is no danger and a clever demon making it seem otherwise.

Is this possible? — Kranky

Yes it's possible but it's not probable. It's massively unlikely.

You should be more afraid of speeding cars than you are worried about deceptive demons.

This notion of not being able to be certain about anything at all, including our own thought/belief...

Where did it come from?

Where did it come from? — creativesoul

Big question, probably requiring more than one discipline to address.

It can be observed that solipsism, the Cartesian thinker, etcetera, all start with the experience of oneself as the beginning of knowledge because that experience is what is given to oneself by default. They equate the immediacy of that necessity with a datum as used in the context of their understanding of the world. The idea being that, if there is a map and a territory, this immediacy is the one location that can be declared impossible to get wrong.

The isolation being imagined is only possible becaue they transpose a use of language that can only arise in the intercourse of life into a conversation where it has no referent.

The absence becomes a pin in the map.

So... the idea itself comes from thinking about one's own thought/belief?

The possibility comes from our experience with ourselves as what cannot be gotten away from. We keep showing up in every movie. I am trying get some space between that experience and any particular "thought" or "belief" that refers to it.
Let me express my observation in a different way. If the skeptic is to question if they know what they are in the habit of thinking as known, why accept the proposition that their experience of themselves as a conscious person is information about their existence that is more "real" than something else? How did we learn to compare things? Is that also "given" in that primary experience of being stuck with ourselves?
Not that I can tell. Your results may vary.

We can be certain of our own abstract thoughts but thoughts based on the environment around us inherently rely on our senses so we can never be completely certain about them.

In talking with others we can be certain we are speaking to separate logical entities but still cannot deduce anything about the environment and our location in relation to them.

There is no such thing as certainty. Can we be certain the sun will rise tomorrow? No. You have to live life based on probability. Most of your fears are 1000 to 1 shots - or 1000000 to 1. Your knowledge may be subjective and open to illusion, but you have to trust that if you havent heard about any demons creating oncoming cars, and nor has anyone else you know, you're pretty safe in accepting the evidence of your senses. And as over the years your experience grows you can become more confident of your intuitive judgement about life's vents.

This causes me anxiety. How can I live my life (if I even exist) doubting that every thought that enters my mind as being real and true. — Kranky

What you have to explore is why you care about certainty to that extent. A psychologist or psychiatrist may be able to help you figure that out.

So if it seems that I'm thinking something, I'm thinking it. End of?