It's on this basis that Spinoza attacks the doctrine of final causes. It's an old argument: since everything happens of necessity, final causes are otiose. Spinoza goes further, however, and suggests a psychological mechanism for the origin of the doctrine of final causes. Because we human beings have a tendency to see the world entirely insofar as it relates to ourselves, we tend (1) to anthropomorphize the world; and (2) to evaluate everything that exists or happens according as it is convenient or inconvenient for us. This is how we get into the mindset of thinking that things happen for a purpose, when in fact they happen entirely out of necessity. As Spinoza says of the proponents of final causes, "But in seeking to show that Nature does nothing in vain, that is, nothing that is not to man's advantage, they seem to have shown only this, that Nature and the gods are as crazy as mankind" (Ethics P1, Appendix).
A bit tendentious, of course; but for similar reasons Spinoza has no patience for design arguments of any kind:
I must not fail to mention here that the advocates of this doctrine, eager to display their talent in assigning purpose to things, have introduced a new style of argument to prove their doctrine, i.e., a reduction, not to the impossible, but to ignorance, thus revealing the lack of any other argument in its favor. For example, if a stone falls from the roof on somebody's head and kills him, by this method of arguing they will prove that the stone fell in order to kill the man; for if it had not fallen for this purpose by the will of God, how could so many circumstances (and there are often many coinciding circumstances) have chanced to concur? Perhaps you will reply that the event occurred because the wind was blowing and the man was walking that way. But they will persist in asking why the wind blew at that time and why the man was walking that way at that very time. If you again reply that the wind sprang up at that time because on the previous day the sea had begun to toss after a period of calm and that the man had been invited by a friend, they will again persist--for there is no end to questions--"But why did the sea toss, and why was the man invited for that time?" And so they will go on and on asking the causes of causes, until you take refuge in the will of God--that is, the sanctuary of ignorance. Similarly, when they consider the structure of the human body, they are astonished, and being ignorant of the causes of such skillful work they conclude that it is fashioned not by mechanical art but by divine or supernatural art, and is so arranged that no one part shall injure another.
So Spinoza is not impressed by design arguments for God's existence. There is another type of design argument that doesn't impress him, though, and that is the problem of evil argument; for it does exactly the same thing from the other side. It seems to me that when peopl claim that 'intelligent design' arguments are arguments from ignorance, what they are actually grasping toward saying is that they are what Spinoza calls 'reduction to ignorance' arguments. They usually are not so consistent as Spinoza in recognizing that most problem of evil arguments (e.g., arguments from poor design) are exactly the same kind of argument.