There are numerous writings on the opposed topics of doubt and speculation. 'Doubt' is an incredulousness at certain types of ideas. 'Speculation' is not a credulousness, but it is a way of intuiting what can't be known. The ideas people doubt are not universals, but that doesn't mean there is no effort being made towards creating a universal understanding. If that is taken as an example of the contemporary foundations of doubt "an effort to universalize incredulousness or to define a rational or ideal credulousness, then speculation can be considered a reluctance to have a certain prescribed credulousness/ incredulousness.

In Cartesianism, the credulousness is towards what one cannot doubt, and the speculation is that a demon or more recently an evil genius is influencing you. Can a person wield this double edged sword, or will such a irrational opposition of thoughts result in defeat? Science accepted this demon as object of study and isolation. The philosopher who tries to learn all ideas, has to reconcile the fact there is a universalization of credulousness and incredulousness that is science/ scientism, and the fact that those who believe anything they can doubt may be demonized.

I can doubt anything that is not a direct perceptual observation. However, a direct perceptual observation such as what someone says, can be doubted. If I ask my sweetheart "were you home alone by yourself all day today while I was at work?" The answer will not be sufficient proof, and in the absence of any evidence to the contrary, persistence in negating the assertion would be demonic possession. It kind of is, and definitely has resulted in what once would have been considered demonic acts. There is an external power over people in their relationships, based on an assumed incapacity to doubt based on direct evidence, and another type of speculative thought. It is a touchy subject, but I have noticed ills in the thinking of people who accuse others that "they have been gotten to" or "they are acting under the influence of a certain regime". I have no answer for this personally, but to embrace a philosophical understanding of how the balance of rationalism swings in the favor of certain adherents.