We are beginning siman 4, which discusses a pillow that lost its stuffing. Pillows were made by taking two pieces of material and stitching them on three sides. Then, they were stuffed with soft material and stitched on the fourth side. 
The first question is whether a person can fill up the pillow lechatchilla on Shabbos. For example, if before Shabbos the pillow was not filled, but the fourth side was stitched almost to the end, such that stuffing could be added on Shabbos and it would not fall out so quickly. The second question is whether the stuffing can be put back into the pillow if it falls out on Shabbos. 
 
The Gemara discusses stuffing a pillow, and concludes that stuffing the pillow for the first time is assur, but refilling it is muttar. The question on the Gemara is what is the concern with filling it the first time, and why is it no longer a concern when refilling it. 
Rashi explains that the concern when filling it for the first time is makeh bepatish, because the item goes from being two pieces of cloth to becoming a pillow. If makeh bepatish is the concern, it does not belong here in our Klal. However, there is a second explanation, which is that it is assur due to the concern shemah yitfor, a person will come to stitch the pillow closed. According to this opinion, the act of filling the pillow is not significant enough to be considered makeh bepatish, so there is no makeh bepatish concern. Rather, the concern is a person will get carried away and sew the pillow shut. However, once it has been filled and not fully sewed closed, since it was already made into a kli, Chazal were not concerned that a person would come to sew it when restuffing it. 
 
One has to assume that it was common enough for the stuffing to fall out, such that it is not considered as though the kli completely lost its status of a kli when the stuffing fell out, because otherwise restuffing it would be akin to stuffing it for the first time according to Rashi, who holds the concern is makeh bepatish.
 
According to Rashi, Chazal did not prohibit stuffing the pillow due to shema yitfor, because there is already a deoraysa concern of makeh bepatish. However, it is hard to understand why, according to Rashi, there is no issue of shema yitfor when resewing the pillow.
 
 
According to the approach that the problem is shema yitfor, when Chazal make a gezeira, they prohibit it due to the natural tendencies of how people act. Thus, over here, Chazal prohibit stuffing the pillow for the first time, because it is natural to sew a pillow right after stuffing it. However, if the stuffing falls out, apparently, people were comfortable leaving the pillow open for a while before resewing, so there is no concern one would sew it on Shabbos. .
 
The Chayei Adam writes the concern of this pillow is shema yitfor, and adds that some hold it is makeh bepatish. It is not clear why the Chayei Adam assumes that the primary concern is shema yitfor rather than makeh bepatish. The Mishnah Berurah only mentions makeh bepatish.
 
Summary
A pillow cannot be stuffed for the first time on Shabbos. However, if it was already stuffed, and the stuffing fell out, it can be restuffed on Shabbos.
 								
				 
															 
								 
			