Quote from Al-Shahrastani [Milal p. 67, II. 2-3], as mentioned by Harry Wolfson in his work The Philosophy of the Kalam
The assumption of an attribute which can be described neither by existence nor by non-existence is the assumption of something which is in the middle between existence and nonexistence, between affirmation and negation, but this is something absurd
I found the statement in the chapter on Muslim sects of creeds when addressing those who differ in the topic of Allah's attributes (sifaat plural of sifah صفة) and here especially the 'Asha'arites.
The statement is part of a quote of imam al-'Asha'ary in which he is addressing those who deny the attributes of Allah.
The quote is as follows in his bool al-Milal wa an-Nihal كتاب الملل والنحل:
فإن إثبات صفة لا توصف بالوجود ولا بالعدم إثبات واسطة بين الوجود والعدم، والإثبات والنفي، وذلك محال، (Source)
However "assumption" in the translation is rather based on the discussion or context, while a-Sharastani speaks about:
إثبات
Which means
confirmation or verification
The full quote with context is the following (starts in page 94 -see here-):
In the following I'll try to translate from Arabic language as these translations are of my own take them with the necessary care!
قال الأشعري: الإنسان إذا فكر في خلقته، من أي شيء ابتدأ، وكيف دار في أطوار الخلقة طورا بعد طور حتى وصل إلى كمال الخلقة، وعرف يقينا أنه بذاته لم يكن ليدبر خلقته، وينقله من درجة إلى درجة، ويرقيه من نقص إلى كمال،
Al-Asha’ari said: If a person thinks about his creation, from what he began, and how he went (passed, literally rotated) through the stages of creation, one after another, until he reached the perfection of the creation, and he'd knew for sure that by himself, he would not manage his creation, transferring him from one degree to another, and elevating him from deficiency to perfection.
علم بالضرورة أن له صانعا قادرا، عالما، مريدا، إذ لا يتصور حدوث هذه الأفعال المحكمة من طبع لظهور آثار الاختيار في الفطرة، وتبين آثار الإحكام والإتقان في الخلقة.
He would necessarily know that he has a Creator, who is capable, knowledgeable, and willing, as the occurrence of these precise acts is not imaginable of nature, due to the appearance of the effects of choice in the (naturally born) instinct (Fitrah), while the affirmation of the effects of perfection and precision in creation became clear.
فله صفات دلت أفعاله عليها لا يمكن جحدها. وكما دلت الأفعال على كونه عالما، قادرا، مريدا، دلت على العلم والقدرة والإرادة، لأن وجه الدلالة لا يختلف شاهدا وغائبا.
As He has the attributes indicated by his actions which can not be denied. Further these actions confirm that he is knowledgeable, capable, and aspiring (Willing), they denote knowledge, ability and will, because the evidence from these acts does not differ in the presence or absence.
وأيضا لا معنى للعالم حقيقة إلا أنه ذو علم، ولا للقادر إلا أنه ذو قدرة، ولا للمريد إلا أنه ذو إرادة. فيحصل بالعلم الإحكام والإتقان. ويحصل بالقدرة الوقوع والحدوث ويحصل بالإرادة التخصيص بوقت دون وقت، وقدر دون قدر، وشكل دون شكل.
Also, there is no real meaning for the Knowledgeable except that he has knowledge, nor for the able one except that he has power, nor for the seeker except that he has a will. By knowledge (only) precision and mastery occurs. It is achieved by the ability to happen and to occur, and by the will to allocate a time without (another?) time, a predestination without a predestination, and a form without a form.
وهذه الصفات لن يتصور أن يوصف بها الذات إلا وأن يكون الذات حيا بحياة للدليل الذي ذكرناه.
It is not imaginable that these attributes describe the Essence is described unless this Essence is alive with life for the evidence that we mentioned earlier.
وألزم منكري الصفات إلزاما لا محيص لهم عنه، وهو أنكم وفاقتمونا بقيام الدليل على كونه عالما قادرا فلا يخلو إما أن يكون المفهومان من الصفتين واحدا أو زائدا، فإن كان واحدا فيجب أن يعلم بقادريته، ويقدر بعالميته. ويكون من علم الذات مطلقا علم كونه عالما قادرا.
And the deniers of the attributes are bound by an inescapable obligation, which is that you (addressing them) have agreed with us by establishing the evidence that He is a All-Knowing, All-Powerful (omnipotent), so it is not devoid of either the two concepts of the two attributes being one or more, and if it is one, then he must be aware of his omnipotence (Qudrah), and he is able based on his Knowledge. And from the knowledge of His Essence He'd absolutely be knowledgeable (and) capable (omnipotent).
وليس الأمر كذلك، فعلم أن الاعتبارين مختلفان، فلا يخلو إما أن يرجع الاختلاف إلى مجرد اللفظ أو إلى الحال، أو إلى الصفة، وبطل رجوعه إلى اللفظ المجرد، فإن العقل يقضي باختلاف مفهومين معقولين.
This is not the case, so one would know that the two considerations are different, so either the difference is due to the mere pronunciation or to the state, or to the adjective, and its reference to the abstract pronunciation is invalid, for the mind decides to (concludes a difference between ...?) differ in two reasonable concepts.
ولو قدر عدم الألفاظ رأسا ما ارتاب العقل فيما تصوره وبطل رجوعه إلى الحال، فإن إثبات صفة لا توصف بالوجود ولا بالعدم إثبات واسطة بين الوجود والعدم، والإثبات والنفي، وذلك محال، فتعين الرجوع إلى صفة قائمة بالذت، وذلك مذهبه.
And if the absence of utterances was estimated at the beginning, the mind wouldn't doubt what it conceived and invalidate its return to the state, for affirming an attribute that is not described by existence or non-existence is to establish a mediator between existence and non-existence, and affirmation and negation, and that is impossible, so it is necessary to refer to an attribute existing in the Essence, and that is his doctrine.
