Logical positivism claims to lay the foundation for each step as it goes along, in a rational way. Yet in reality it puts forth no theoretical universal to validate its very first step. Positivists accept (though they present no logical reason why this should be so) that what reaches them from the "outside" may be called "data"; i.e., it has objective validity.
This dilemma was well illustrated by a young man who had been studying logical positivism at Oxford. He was with us in Switzerland as a student ... and he said one day, "I'm confused about some of these things. ... when this data reaches you ..."
At once I said, "How do you know, on the basis of logical positivism, that it is data?"
He started again, and went on for another sentence or two, and then said a second time, "When this data reaches you ..."
...I had to say, "No, you must not use the word data. It is loaded with all kinds of meaning; it assumes there is objectivity, and your system has never proved it."
"What do I say then?" he replied.
So I said, "Just say blip. You don't know what you mean by data, so substitute blip."
He began one more, "When blip reaches you ..." and the discussion was over. On the basis of their form of rationalism, there is just as much logic in calling something "blip" as "data."
Thus, in its own way, though it uses the title of positivism and operates using reason, it is just as much a leap of faith as existentialism - since it has no postulated circle within which to act which validates reason nor gives a certainty that what we think is data is indeed data.
Michael Polanyi's (1891-1976) work showed the weakness of all forms of "positivism" and today positivism in theory is dead. However, it must be said that the materialistic, rationalistic scientists have shut their eyes to its demise and continue to build their work upon it as though it were alive and well. They are doing their materialistic science with no epistemological base. In the crucial area of knowing, they are not operating on facts but faith.Francis Schaeffer, "The God who is there", emphasis mine.
The trouble is that there are many non-scientists who have accepted the epistemological assertions of the "materialistic, rationalistic scientists" who "have shut their eyes" the the demise of their epistemological foundation, that science is an adequate philosophical foundation for not believing in God. "Well, we know so much more than we used to know. It used to be necessary to believe in God to explain the world around us. But nowadays, we are much better informed, and belief in God is not necessary."
Science as a philosophy - "scientism", if you like - is not built on a solid foundation. For example, Richard Dawkins said: "Although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist." This is not a logical statement. Firstly, although Darwin provided a naturalistic and gradualistic explanation of how life might arise, this actually has no bearing on whether or not there is a god (which is, in effect, what Dawkins is claiming). Secondly, what is absent from Darwin's (and Dawkins') work is reference to an epistemological foundation. It is a justification of this which would provide the possibility to be an intellectually fulfilled atheist, rather than a description of phenomena. Questions such as: how does life differ from non-life? what is consciousness? what is communication? why do the things that matter so much to us - truth, love, beauty, justice - seem to have so little to do with the physical nature of the universe?
This isn't to say that science is bunk. On the contrary, the achievements of science in explaining the nature of the universe are immense and wonderful. Also, some scientists have made sincere attempts to answer these questions. But like the student that Schaeffer talked to, their answers are not philosophically complete.
Science is not the sole preserve of logical positivists. In fact, the foundations of modern science were laid by people with a very different philosophical framework - Christians, who believed that the foundation for belief in the objective validity of data was the existence of a deity, an external absolute reference point. Christians still do science today. It's uncommon for their books to be as successful as those of the logical positivists who haven't comprehended their mislaid foundation yet, though.