Dear K.,
When my daughter was born she was constantly waking up every 2-3 hours. Then around about 2 months old, she began to cry every time that I laid her down and then become quiet when I picked her up. I nursed her and she was hungry every 2-3 hours. When she developed this "crying phase" at that age, I couldn't even lay her down just to take a shower without her crying and this was after I got her to sleep. The moment I laid her down in her crib, she would wake up and start crying. I asked my pediatrician what I should do about it and he told me that she would grow out of it. She was way too young to allow her to just lay there and cry because she wasn't trying to "get her way", she just simply was comfortable being held. The crying thing only lasted for about a month and then went away.
As for your 6 month old, I don't think allowing him to cry is the answer. He is far too young to understand the concept of control just yet, but he's close. With that said, I would look more toward the issues of comfort, perhaps hunger even though you might think he is full, and also stomach problems. Since he cannot verbalize to you what the "problem" is, the only thing that a baby can do is cry to let you know something is wrong in his little world. If babies could come out of the womb talking it would make our lives a little less stressful, but that is not the way God planned it, so you just need to learn which cry is for hunger, which cry is for soiled diapers, and which cry is just because he wants to be close to you. Letting him cry it out right now is not healthy because you could be masking a need that he has and is looking for you to fix or fulfill. Six months old is too young to manipulate you with crying, so I would tend to believe that it is something else that he needs.
Hope this helps.
T.