There are few figures in the history of philosophy equal to or more significant than the German philosopher Immanuel Kant (1724-1804). Kant did important work in numerous branches of philosophy, including especially groundbreaking work in moral philosophy and epistemology.
The main thrust of his work in epistemology, as laid out especially in his The Critique of Pure Reason, is an effort to construct a theoretical compromise between the position that our ideas do not connect with a real world independent of them and the position that our ideas are copies of a real world independent of them.
In pursuit of this goal, Kant draws an important distinction between the "noumenal world" and the "phenomenal world." The noumenal world means reality as it is, and the phenomenal world means reality as it is perceived. Thus they are not two "different" worlds (a common mistake newcomers to Kant's philosophy make).
According to Kant, our minds shape incoming sense-data into certain broad categories. For instance, we of necessity see everything as located in three-dimensional space, moving through absolute Newtonian time, and fully explainable in principle by a set of natural causal laws.
That is to say, everything in our phenomenal world (i.e., the world as it is perceived by us) will have those characteristics, for anything else would be literally unthinkable. But it doesn't follow that the noumenal world (i.e., the world as it is in itself) has to have these characteristics, and in fact such characteristics probably would not even make any sense outside the context of the phenomenal world. Nor does it follow that the phenomenal world of other types of beings with different types of minds and sense-organs (i.e., the world as it is perceived by them) will have these characteristics.
Any analogy unfortunately can only be quite loose, but imagine if red-tinted lenses were permanently attached to everyone's eyes. Visible reality to us would be all and only different shades of red. Not because everything really is red, but because the way we process it forces it into that category.
Or even more extreme, think of the relation between language and the world, between the word "horse" and a horse. There are things we can say of "horse" (such as that it has an 'h' in it) that are not true of horses (they don't have any 'h's in them).
If we were to say that a certain object was a particularly dark shade of red in the first case, or that "horse" had more letters than "pig" in the second case, it's not that those claims would be false. As long as we mean that as seen through these permanent red-tinted lenses the object is a dark red, or the word used to refer to a horse has more letters than the word used to refer to a pig, we're fine. It's only if we claim that the object itself is dark red, or actual horses have more letters than actual pigs, that we manifest a confusion.
To say that the noumenal world is a world of space, time, and cause and effect would be like saying horses have 'h's in them. 'h's don't exist in animals. They aren't even the type of thing that exists in animals. They exist in language that is used to represent, among other things, animals. To attribute our phenomenal categories of thought to reality as it is in itself would be that kind of error.
So is Kant saying space, time, and causation and such are illusions? No. No more than it's an illusion to think "horse" has five letters. Again, as long as it's understood that we are referring not to the world independent of our perception (the noumenal world), but instead to the intersection of that world and us (the phenomenal world), then such notions are not illusions at all.
Can we only, then, speak of the phenomenal world, or are there things we can know of the noumenal world as well? There's really next to nothing we can know about the noumenal world (reality as it exists in and of itself). We can say that it exists, and we can say that the result of our interacting with it is the phenomenal world, but that's about it. We can never somehow perceive it independent of our perceptions to see how it compares to the phenomenal world, because if we were perceiving it, then it obviously couldn't be independent of our perceptions.
Is Kant right? Well, in a broad sense he's probably on to something, but not surprisingly his ideas have been challenged strongly over the years and are unlikely to be true down to every detail.
For example, advances of modern science have already called into doubt some of what Kant thought were limitations on how we can conceive the world. Relativity has changed our notions of Euclidean space and Newtonian time, and quantum mechanics has replaced our notion of causal determinism with a probabilistic form of causation. Certainly these ideas are counter-intuitive and very difficult for most folks to grasp, but they are evidently not literally unthinkable to the human mind.
Whether these developments refute Kant's ideas, necessitate a small adjustment in them but otherwise leave them standing, or do not conflict with them at all depends on one's interpretation of Kant's philosophy. This and much more about Kant's epistemology continues to be debated amongst philosophers.