Since I work with students best described as "consistently inconsistent" I frequently find myself going around and around with well meaning colleagues on the idea that students need to "prove" that they know A or B (usually vocabulary.) (As if any typically developing child is required to "prove" their knowledge of every vocabulary word they can utter.) We usually get stuck because most of my students will not consent to participate in assessment-style activities. They will produce inconsistent or meaningless responses because they simply cannot be motivated to identify a "fork" from a field of 4 pictures. And so, the skeptics tell me I cannot "assume they have the skills:" I have to teach them.
By presuming competence, I refuse to do either. My teaching does not assume that my student can identify a picture of a fork (or numbers, or whatever other vocabulary is in question.) Nor do I spend my time direct teaching basic pre-school vocabulary. I can teach the 8th grade math curriculum (geometry and equations) without knowing for certain if my student knows number symbols. Will I teach number symbols in the process? Absolutely. I can teach mid-grade literature without knowing if my students can identify so-called "functional" vocabulary or know what a "wh" question is. Will they learn that in the process? Probably. They'll also read some really good literature that is appropriate to their age. (Please don't get me started on "wh" questions - I have found that when most people say a student doesn't know "wh" questions they really mean the student doesn't have a certain level of general knowledge, which is generally to be expected of students with complex disabilities and fundamentally Not. The. Same. Thing. One is skill, the other is content. Can you guess which one I care about more?)
My students, like all other students, will use vocabulary to answer questions and create assignments. That will tell me what they know. I don't need them to identify pictures on an assessment they don't care about. I need them to use them in a meaningful context. My students, for whom formal language continues to be a weakness, will demonstrate comprehension of concepts in a myriad of non-linguistic ways, and I will accept those as equally valid measures of their comprehension. Because I understand that, especially for students just learning formal and symbolic language, the symbolic representation and the concept are not the same thing.
That is what Presuming Competence means to me. It means not letting the fact that I cannot prove whether or not a student knows a concept or has a skill through formal assessment hold me back from teaching them higher level materials. Simply put, it means believing that all students can learn and teaching them.