There is a world out there, and your senses do give you an impression of it which yields genuine knowledge of empirical objects, according to Kant, but what empiricists fail to take into account is the apparatus that we depend upon to make sense of that world:

“Kant thought that Berkeley and Hume identified at least part of the mind’s a priori contribution to experience with the list of claims that they said were unsubstantiated on empirical grounds: “Every event must have a cause,” “There are mind-independent objects that persist over time,” and “Identical subjects persist over time.” The empiricist project must be incomplete since these claims are necessarily presupposed in our judgments, a point Berkeley and Hume failed to see. So, Kant argues that a philosophical investigation into the nature of the external world must be as much an inquiry into the features and activity of the mind that knows it” 

As mentioned, The American thinker W.V.O Quine has a dispute with the way in which Kant arrives at his answer to that problem. 

From a paper by Arthur Sullivan here:

“There do not exist two distinct types of reality in the world which require two distinct modes of expression. This leads Quine to conclude that the analytic-synthetic distinction is a purely logical convention that is ontologically unnecessary and empirically superfluous. In this respect, Quine agrees with the radical empiricism of Mill, with its claim that there is no a priori knowledge. The fact that something is the case, or even the fact that something seems to be necessarily the case, does not imply the reality of a priori truths. Quine goes so far a to refer to the notion of a priori knowledge as a “metaphysical article of faith.”

Of course, so also did Schopenhauer have a problem with Kant (wikipedia summary here).

This quote was found here:

“Empirical concepts are ultimately based on empirical perceptions. Kant, however, tried to claim that, analogously, pure concepts (Categories) also have a basis. This pure basis is supposed to be a kind of pure perception, which he called a schema. But such an empiricist analogy contradicts his previous rationalist assertion that pure concepts (Categories) simply exist in the human mind without having been derived from perceptions. Therefore they are not based on pure, schematic perceptions.”

Just some thoughts on a Sunday, as it was requested by a friend.  If you can refer me to a more comprehensive critique, I’d appreciate it.