I exist?
God?
Thought?
I can doubt all of these things. There is nothing you or I cannot call into doubt or disbelieve.
I can even doubt my own doubting.
Right… then there is no way of finding out what is left. It’s finished. Thank you, have a nice day!
Well, we can doubt everything if we want, but it doesn’t mean that what we doubt doesn’t exist.
For example, I can doubt that I am reading words on a computer screen, but if I use my eyes I can clearly see words are there.
a = a
try doubting that, see how far you get.
You cannot doubt that pain is pain. Pleasure may be the relieving of pain, the relieving of some unconscious anxiety or tension. But pain cannot be doubted.
If you don’t have a particular idea you can’t doubt that idea. You can only doubt ideas that you have in your mind.
Pain has this pesky habit of overriding the thought upon which any doubt would be dependent anyway.
You shouldn’t really doubt the existence of thought because doubting is a thought in in itself, so your doubting the existence of thought would be illogical. But then again, you can doubt whatever you want because nothing is stopping you from illogical thinking.
In a way, though, you can doubt your own doubting or thought by questioning who or what is actually doing the thinking or doubting. Also, you can doubt the content of the doubt or thought, whether it is valid or coherent, like that.
That is true although that refers to specific thoughts and not the existence of thought in general.
But you can doubt that what you think is your thoughts as being your thoughts and not the implanted thought of another thinker. Then again doubt that the doubt is the thought of you or another thinker, then doubt that that doubt of a doubt is the thought of you… ad infinitum.
You can also wonder whether you are accessing the thoughts of another or of a greater mind.
Of course it’s possible to doubt anything. I once taught basic maths to a girl who doubted that 1+1=2. This is a psychological matter.
A more interesting question is whether there are any propositions which are such that believing them is sufficient for their truth. One candidate is the proposition that there is a belief. Try believing that falsely.
My love for my children…there is Not a doubt in my mind and never will be!
What about the fact that at least in this solar system, the planets revolve around the Sun. I thought that we at least proved this…or are we to go back in time in our arrogance to doubt and to believe that we are at the center of it and of the entire universe? Poor Copernicus! 
Aren’t there any scientific theories that have become fact…through much experimentation, gathering of evidence - to have become emphatically and unquestionably proven.
Even at some point, if something is ‘proven’ to be wrong by some flux/fluke/random accident or discovery, 
it isn’t even a question of doubt there. It’s knowledge we’ve gained and then on to discovery…again. I am not here speaking of unproven theories…or even of quantum theory, which for many has been proven but still…
Pessimistic metanalysis based on past falsity…nah…just because something[s] turned out to be false that was/were once considered true or truth doesn’t mean that we ought to automatically doubt everything that is known/proven to be true now and assume that it will be proven wrong at some point.
By doing that, by using “I” to do anything only strengthens “I”. When thought is not there, “I” is not there.
…and then again…on the other side of that coin…
the real "I’ is given the opportunity to show itself, to come to the forefront. 
Pretty far actually.
This is an excerpt take from Leon Trotskys “The ABC’s of Materialist Dialectics” countering the vulger materialists approach to logic.
I will here attempt to sketch the substance of the problem in a very concrete form. The Aristotelian logic of the simple syllogism starts from the proposition that ‘A’ is equal to ‘A’. This postulate is accepted as an axiom for a multitude of practical human actions and elementary generalisations. But in reality ‘A’ is not equal to ‘A’. This is easy to prove if we observe these two letters under a lens—they are quite different from each other. But, one can object, the question is not of the size or the form of the letters, since they are only symbols for equal quantities, for instance, a pound of sugar. The objection is beside the point; in reality a pound of sugar is never equal to a pound of sugar—a more delicate scale always discloses a difference. Again one can object: but a pound of sugar is equal to itself. Neither is this true—all bodies change uninterruptedly in size, weight, colour, etc. They are never equal to themselves. A sophist will respond that a pound of sugar is equal to itself “at any given moment”.
Aside from the extremely dubious practical value of this “axiom”, it does not withstand theoretical criticism either. How should we really conceive the word “moment”? If it is an infinitesimal interval of time, then a pound of sugar is subjected during the course of that “moment” to inevitable changes. Or is the “moment” a purely mathematical abstraction, that is, a zero of time? But everything exists in time; and existence itself is an uninterrupted process of transformation; time is consequently a fundamental element of existence. Thus the axiom ‘A’ is equal to ‘A’ signifies that a thing is equal to itself if it does not change, that is, if it does not exist.
At first glance it could seem that these “subtleties” are useless. In reality they are of decisive significance. The axiom ‘A’ is equal to ‘A’ appears on one hand to be the point of departure for all our knowledge, on the other hand the point of departure for all the errors in our knowledge. To make use of the axiom of ‘A’ is equal to ‘A’ with impunity is possible only within certain limits. When quantitative changes in ‘A’ are negligible for the task at hand then we can presume that ‘A’ is equal to ‘A’. This is, for example, the manner in which a buyer and a seller consider a pound of sugar. We consider the temperature of the sun likewise. Until recently we consider the buying power of the dollar in the same way. But quantitative changes beyond certain limits become converted into qualitative. A pound of sugar subjected to the action of water or kerosene ceases to be a pound of sugar. A dollar in the embrace of a president ceases to be a dollar. To determine at the right moment the critical point where quantity changes into quality is one of the most important and difficult tasks in all the spheres of knowledge including sociology.
Every worker knows that it is impossible to make two completely equal objects. In the elaboration of baring-brass into cone bearings, a certain deviation is allowed for the cones which should not, however, go beyond certain limits (this is called tolerance). By observing the norms of tolerance, the cones are considered as being equal. (‘A’ is equal to ‘A’). When the tolerance is exceeded the quantity goes over into quality; in other words, the cone bearings become inferior or completely worthless.
Our scientific thinking is only a part of our general practice including techniques. For concepts there also exits “tolerance” which is established not by formal logic issuing from the axiom ‘A’ is equal to ‘A’, but by the dialectical logic issuing from the axiom that everything is always changing. “Common sense” is characterized by the fact that it systematically exceeds dialectical “tolerance”.
Vulgar thought operates with such concepts as capitalism, morals, freedom, workers’ state, etc as fixed abstractions, presuming that capitalism is equal to capitalism. Morals are equal to morals, etc. Dialectical thinking analyzes all things and phenomena in their continuous change, while determining in the material conditions of those changes that critical limit beyond which ‘A’ ceases to be ‘A’, a workers’ state ceases to be a workers’ state.
The fundamental flaw of vulgar thought lies in the fact that it wishes to content itself with motionless imprints of a reality which consists of eternal motion. Dialectical thinking gives to concepts, by means of closer approximations, corrections, a richness of content and flexibility; I would even say “a succulence” which to a certain extent brings them closer to living phenomena. Not capitalism in general, but a given capitalism at a given stage of development. Not a workers’ state in general, but a given workers’ state in a backward country in an imperialist encirclement, etc.
I can doubt myself, my I, and my ego. The words do not even reflect what I am, so of course I can doubt the concepts.
Something will happen because there is energy.
That energy is something which cannot be defined and which cannot be understood. Not that I am mystifying it. The moment the dead thought tries to capture that life energy, thought is destroyed. Thought is matter. The moment it is created, it has to be destroyed. But that is the very thing that is resisted. Thought is born and is destroyed, and again it is born and again it is destroyed. The only to can give continuity to thought is through the constant demand to experience everything. That is the only way of trying to maintain the continuity of the `experiencing structure’.
What can be said of ‘self’ other than what is conceived of ‘self?’ Also, What is there to use to doubt? Another ‘self’ of which is not conceived?
Why do you presume that doubt is a thought?? I can doubt that doubt is a thought, too.