OK, Firstly the idea behind this is of my opinion, but is backed up by solid fact, so i wouldn't say all other ideas that there are other causes of the Renaissance are wrong, since it is just a matter of opinion.
The reasoning goes.
An oft overlooked aspect of the renaissance that we always choose to overlook is the concept of banking and the rise of the wealthy non-aristocrats. This would lead to the repellation of sumptury laws in the 16th/17th century (300 years after the renaissance).
Truthfully when we look at the renaissance we only remember the art or the artifacts. But really the main thrust for modernization (IMO) was the idea of private ownership of wealth. There was a sense of private ownership in the middle ages, but it was only limited to landed gentry, and even they had to pledge fealty. But with increase in trade, the idea of actually owning luxuries grew. The wealthy merchant class rose in influence and with that they could commission or patronize artists. As much as we like to think that genius pays for itself, if Michelangelo was never paid to do the fresco or David, would he have done it?
Anyway, it wasn't spices they traded. Paper gunpowder block printing, all these ideas and inventions stem from the east, and only when it was safe to use the silk road were these ideas spread.
Fastpawn, I certainly don't disagree with you on the fact that one of the main sparks of the renaissance was economic. Although 'thought' at the time was changing, there is no way it could have gained the widespread diffusion it did without the support of wealthly patrons. Thus, it's important to look into the underlying causes of this growth in wealth.
The italian renaissance was, of course, the first of many 'rebirths' in european countries, and historians have isolated the merchants of italians coastal cities--such as venice--as the ones to get this accumulation of wealth started. But how did this wealth come to them? You mentioned one possible reason--the opening up of china--but the Renaissance, which IMO is almost too big and complex to characterize as a single period, demands a wide scope of causes.
So, going back to the question: what brought this wealth (and ideas, as you mentioned) to italy? I think one would have to begin with the wars with the east (middle east). Starting with the expulsion of the Mulsims from Sicily in (circa) 1012, and continuing on into the first crusade of 1095 and the later crusades, the christian west slowly opened up the mediterannean, which for years had been a de facto muslim lake, for her ships and tradesmen. After the first crusade, indeed, the europeans gained a foothold in the middle east as well. The merchants now had a sea to play in, and they were further helped by the vast influx of wealth paid to them by crusaders and normal pilgrims who needed to be transported to the holy land, byzantium, or simply across the adriatic.
This now established the connection with the east that your theory demands. Moreover, some of the intellectual basis comes from this, like you mentioned, with intellectual remnants from the great classical ages being brought back from byzantium.
The europeans gradually pushed eastward more into the middle east, and I guess you could say the mongols 'met them half-way'. However, i always saw the connection between the west and china as an indirect one, with merchants of the middle east linking the two. Of course, we all know about marco polo, but i'm not sure of the extent to which european travellers went into china--I don't necessarily deny it, i just haven't researched much into it, so perhaps you could tell me more about it.
Anyways, i think the major accumulation of wealth came, at least at first, from the merchants and shipsmen of italy, not the travellers and traders of the middle east and china. Moreover, the middle east was quite wealthly at the time, so it's hard to say whether or not europe needed china for the renaissance to happen. Also, could you tell me a little more about the time period during which china opened up to the middle east/europe? I'd like to see it in relation to the dates of the growth of the renaissance in italy and the rise new wealth.
What you also mention are a lot of the innovations and ideas that came from china to europe. But i see the renaissance as more of an intellectual growth, and a changing of a way of thinking, rather than a technological development. Guttenberg's printing press made ideas widespread, but are we certain that he got his idea from china? (I know it existed in china before, but that doesn't mean guttneberg didn't invent it independently). Moreover, the development of printing came much after the beginnings of the renaissance, so printing wasn't probably wasn't necessary, although without it the Renaissance wouldn't have been nearly as far-reaching. Also, gunpowder helped, but a change in warfare didn't do much to impact the renaissance.
Anyways, another factor on which the economic causes of the renaissance are based is the economic and societal changes that occured during the period. What i mean is the growth of an actual money economy. During the middle ages, the only real form of wealth was land, but by the start of the renaissance there appeared a money economy, one that made it much easier to make transfers and to gain wealth. This in turn contributed to new private ownership, which you mentioned. Moreover, in some places, cities began to grow and opened up whole new industries, in place of than the old agragrian society. All of these led to the growth in wealth of the patrons and more trade and idea diffusion. But the underlying causes of these things are even more difficult to assess...
I've outlined only a very limited amount of what i think to be the causes of the renaissance, neglecting ones such as the decline of the church, competition among city states, etc. The Renaissance is an extremely difficult period to assess, and any attempt to (IMO) requires an array of causes, chain reactions, etc. I think your theory had something to with causing the renaissance, but i don't see it as a proximate cause, i don't know if the renaissance depended on its happening, and, like i said, IMO the renaissance as a period requires an array of causes. That said, i think you have an interesting idea, one that doesn't get too much coverage, and i'd certainly like to hear more about it if you're willing.
Also, if i misunderstood your ideas, then let me know, because i personally hate when other people just ignore or misunderstand my thoughts and go on to post whatever they think as their response, even if it doesn't deal with my post at all. I wouldn't want to be one of 'those guys'.