Idealism and realism are different in concept from that between optimisim and pessism, even though I sometimes here people say, “I’m not pessimistic, I’m realistic”, as if the ideas were similar.
Idealism and realism concern perceptions of reality. An idealist will see the world as he wants it to be, whereas a realist sees it as it is. I think that idealism is great for vision, but terrible for planning and implementation. The same – conversely – with respect to realism. Both of these traits are good, if they balance each other and are applied at the right times.
Pessimism and optimism concern our expectation of outcomes. That is, any endeavor which reaches into the future touches on the unknown. The unknown is full of surprises, both bad and good. The optimist will prefer to anticipate a positive outcome. If he is also a realist, he will acknowledge the possibility of a bad outcome, but will focus on and anticipate the positive one. The idealist will ignore the bad outcomes entirely, and fail to plan for them.
The pessimist on the other hand will prefer to anticipate a negative outcome. He simply assumes that most endeavors are bound to fail, or will not achieve their highest fulfillment. If he is an idealist, he will never believe that good things can happen; if he is a realist, he will accept this possibility, but still plan for the negative.
So when people say, “I’m not being pessimist, I’m being realistic”, it confuses the meaning of these two words, which apply to different elements of perception.