I do have some additional thoughts on this question, which is a good and important one. These involve logical constructs, perspective, the vagaries of evidence and memory, and our human need but imperfect need to make meaning through the imposition of narrative.
First, some discussion about deductive versus inductive reasoning might be helpful to shed some light on the study of history within the context of assumptions. Deductive reasoning is going from the general to the particular, positing a hypothesis and then finding particular means of supporting it, as the response above describes. Inductive reasoning means going from the particular to the general. Both are perfectly valid means of drawing conclusions, but both have limitations that anyone who studies history must be aware of.
As discussed above, when one creates a hypothesis and then goes looking for the means to support it, unlike in a scientific inquiry, where one has a control group and can isolate a variable to investigate, in history, quite often, there is no handy control group and variables frequently cannot be isolated. If I hypothesize, for example, that without Hitler, there would have been no World War II, I have no way of constructing an alternative universe to show this. Nor am I at all inclined to "see" any evidence that supports a contrary position. So, the drawbacks in history for deductive reasoning are that there is no control group and one is predisposed to go looking for only evidence that supports one's hypothesis.
In inductive reasoning, going from the particular to the whole, we have a tendency to forget that not all evidence is available to us. We can never know all the actors, events, and circumstances that might inform our conclusions. Thus, if I find a box of letters in a collection from a soldier in World War I from one side of a battle, whatever I construe from that box of letters may never be balanced by a collection of letters from a soldier on the other side of the battle. So whatever inferences I care to draw should always take into account this problem.
As a general matter, the availability of evidence in constructing history is problematic. What remains? What has been carefully preserved? What has been tossed on the trash heap? What is out there that we still do not even know exists? We find what we find and do the best we can with it, but the further removed people and events are, the less likely it is that we can have a complete picture of anything.
How we remember and record events is similarly fraught with difficulty. The more contemporaneous an account is, the more likely it is to be accurate, but we know now from a great deal of research that memory is imperfect. An additional difficulty is that accounts can be and often are completely self-serving. When we write something or give oral testimony that is then recorded, we have a tendency to always put ourselves in the best possible light. This is as true of farmers as it is of presidents. So, primary sources must be viewed with this in mind.
Perspective is another important aspect of the study of history. Those who study it often have a point of view that clouds the ability to consider other points of view. There are those who argue that geography is the overriding factor that influences history, and there is ample evidence to show this. There are those who think that it is individuals who influence history the most, and there is evidence to show this as well. The problem is that in each instance, the historian has on blinders that can preclude other perspectives. I do not want to say that all history is propaganda, but certainly, every historian has an axe to grind, and his or her selective perception cannot help but hold sway to some degree.
Finally, history is just "stuff" that happens. And it's very messy. We have a need to impose order on what happens and to make meaning of it, to give purpose to existence. We impose a narrative on our own lives in this way, and we impose a narrative on history in this way. Is this actually history? Is this a false construct? Is this six blind men describing the elephant? I would never want to discourage anyone from the study of history. It is valuable and enriching in any number of ways. But I do think it is important to understand its limits.
As any academic discipline, history has its own unique subject matter and methodology. While there are many different ways to study the past, not all of them are considered history.
History by its nature offers up a coherent narrative about the past, including not just a list of events but some sort of explanation for the events and their significance. A list, for example, of all the Kings and Queens of England would not be a history, but a "chronicle".
Instead, history makes the assumption that the past can be interpreted by a careful study of multiple sources. Each interpretation of some element of the past is shaped by certain choices of question. While some historians are interested in political or military history, other are more concerned with religion, economics or daily life.
Most of historical method is a matter of careful collection and analysis of sources to provide evidence supporting a particular hypothesis or interpretation.