Revising your writing is always a good idea, even if your instructor doesn't ask you to. It improves your philosophical work and is a great skill to practice for your other writing too. The downside is that it can be hard and stressful! There a funny piece here about how daunting even professional writers find the revision process, if you need some moral support.
Sometimes your instructor will require you to revise and resubmit a paper you've written for the course. They will give you comments on the main areas for improvement and ask you to rewrite your paper in light of those comments and the new thinking you've done in response.
The new paper will be held to a higher standard that the original version, with the result that sometimes, despite genuine improvement in the paper, any existing grade you have won't be raised. If the revised paper meets the following conditions, though, a higher grade is at least likely:
The paper must be substantially revised, throughout. Changing the odd word or phrase, correcting spelling or grammatical errors, deleting or relocating a few sentences, or inserting a single new paragraph aren't enough.
The paper must evidence substantial new thought on your part. Your instructor will be looking for brand new material that you yourself have come up with. Simply incorporating the comments that you've received into the original text won't be adequate. Relatedly, don't assume that the parts of your paper that haven't been commented on don't need revision. Often changing one part of a paper will require follow-on changes in other parts, and it's not your instructor's job to point each of those out to you.
The paper must be significantly better. It must show improvements in clarity, structure, comprehensiveness or argumentation. Change alone won't get you far.
The paper must respond to a significant number of the suggestions that were raised in the comments. You aren't required to change your paper in all of the respects that your instructors suggests: you might think your paper is better as it stands in regard to some of them, or could be improved in a different direction than the one suggested. You do, though, need to carefully consider all the points your instructor has made. They generally have a pretty good idea of where the most important areas of improvement are and will be checking that you've at least taken note of their comments in the second version of your paper.
Note: while you will hopefully get a revised grade for your second draft, you shouldn't expect to get extensive written comments the second time round. Some effort will be made to explain why the grade was raised (or not), but if you want more substantial feedback or advice, you'll need to send an email or make an appointment.
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