Their view, briefly:
(1) it is possible to know things as they are, (2) through mystical insight (and not through logical means), (3) there are insurmountable limits to language and logic when it comes to expressing such knowledge.
Islamic belief is based on certainty that there is an absolute truth about things as they are. The least that is expected from a believer is to have faith about an ultimate reality of things as they are.
In Islam, truth is not relative. However, each man's capacity to perceive differs and his manner of expression may differ, about things as they are, or reality as it is, that is the same for all, regardless of how one may choose to express oneself about it. In other words, differences in perceptions and manners of expression are about the same things as they are, but those perceptions and manners of expression, are affected by various backgrounds (cultural, educational, personal intelligence, etc.). Such differences are indications of various levels of sensing the same final reality of things. And such differences are not constant or unchangeable. To the contrary, perception about existence can improve, and with it man's understanding of existence can get better and better.
As for Sufis, they go further than just believing in things as they are. They are certain that it is possible to 'know things as they are'. Yet, regarding the way to attain that level of awareness, Sufis are unanimous that rational approaches will always be deficient compared to a mystical approach that starts from clearing the clutter in one's inner sight and purifying the heart to make it eligible for 'sight'. Preparedness has nothing to do with reading books and one's analytical competence (which is not dismissed or disliked, by the way. See below). Rather it begins with faith, following religious duties, strict control over cravings, tendency to silence, meditation and contemplation. It has to do with developing one's ability to see beyond appearances, an ability honed not when the mind is full of thoughts but when it is silent and calm.
As I understand the Sufi view, a person who has reached the Mind level of awareness (The Hekam: #67) realizes how inappropriate it is to let sensuous matters control one's behavior. In a similar way it seems that Sufis, after reaching the level of mystical insight (starting at the Heart level and beyond), realize that mental reasoning need to be sort of 'reined'. Regarding rational approaches, Sufis like Ibn Arabi use the term 'Proper Reasoning' to describe what I think is reasoning aided/guided by mystical sources of information (Quran, the Prophet's sayings and notes of Sufi scholars' understanding of the two sources). In this case 'proper reasoning, only 'points to' knowledge of things as they are. Sufis note that even in following this approach (proper reasoning), what can be gained is getting 'as close as possible' to know things as they are, for someone without mystical insight.
If I understand Sufis correctly, levels one might be at regarding sensuous matters and logical constructs have a tight link to how far one is advancing regarding mystical insight. The impression I get from Sufi literature is that sensuous desires (considered but restrained), mental capacity (considered but reined), social behavior (gentler), moral values (improved), are different aspects of the same essence of an individual. It is a one bundle with different aspects and different angles to look at. Purification of the essence will have its effect on all aspects. From another angle, seeking to purify the essence has to be associated with steps taken to improve one's position on all fronts. This view of Sufis could be the reason why would later Sufis defend Sufis from former generations, against accusations that do not fit with a multifaceted improvement that Sufi masters have to go through, before they can realize finer meanings (More details in The Hekam: 13 to 16).
As I understand Sufis, any sort of reasoning, using logical frameworks and linguistic and grammatical structures cannot take one's knowledge as deep as mystical insight. Sufis consistently describe a stage of awareness that is reached when those structures become barriers that hinder finer sight of the original ideas, that those structures are only trying to represent (related discussion in The Hekam #42). And any representation of the pristine ideas through language and logic cannot be the same as 'seeing' the ideas themselves. Hence the need to silencing thoughts as a necessary step to realize what is beyond them.
Sufis see Grammar and Logic as 'aiding tools' to express knowledge, and not tools that acquire new knowledge. One knows through sources of 'original information': instinct, senses, and mystical insight. That is when information enters the circle of one's awareness. Only after that, one tries to organize his thoughts, express oneself, and formulate his knowledge in a communicable form, and that's where tools to express oneself come in handy. If something is totally unknown, no amount of using 'tools of expression' can bring it into one's circle of awareness.
It is 'tasting it' rather than 'analyzing it' that Sufis urge. One of the analogies they give, is that of describing what Honey tastes like to someone who has never tasted a sweet thing. Even if the listener became convinced about what he was told regarding Honey and how it feels to taste, it is never the same as actually tasting honey. Concerning 'knowing things as they are', Logic leads to a dead end because using logic is similar to describing how it feels to taste honey. While mystical insight is equivalent to actually tasting honey.
Given the above background, I would guess that the Sufi view of much of what a scientifically minded person describes as 'discovering' would most likely be considered 'further exploration' of a bulk of information that is already available but not fully sorted out. In this case, tools of expressions are helping us to express our improving understanding of what we already are familiar with, and to clear the vagueness surrounding it.
Reasoning can be deductive or inductive. In the first case, the outcome is already embedded in the premises. In the second case, closely examining a bulk of information that is accessible, helps us detect patterns. In both cases there is no original information rather 'more specific' or 'better expressed' information coming out of sorting out pieces of information that we already know at different levels of clarity. A professor in agriculture might or might not understand farming better than a native farmer. Both are looking at the same thing, the professor is probably more articulate but not necessarily more knowledgeable about farming.
Sufis do not dismiss logic and grammar! Since, logic and grammar are essential in expressing thoughts. There is a sort of a complementary (rather than a rivalry) relationship between Sufi insight on the one hand and logic and grammar on the other. Mystical insight is a source of original information, while logic and grammar are tools to organize thoughts in a communicable manner. That understanding probably explains why a Sufi like Al-Ghazali, while confirming that logic on its own leads to a dead end with regards to 'knowing things as they are', yet, he continued to promote teaching logic and he did write several books on the subject.
Related Links
Further background regarding Sufi Insight and the Rational approach is presented in : Sufis and Kalam
